Alessandra Giangrande

Mauro Viganò

Marco Carbonaro

Marco Gilardone

Giacinto Peppino Tropea

Luigi Cerone

Massimo Corbo

Marco Gazzoni

Alberto Botter
email: alberto.botter@polito.it

Giacinto Luigi Cerone

Swallowing Onset Detection: Comparison of Endoscopyand Accelerometry-Based Estimations

Keywords: swallowing muscles, accelerometry, FEES, sEMG, symmetry indexes

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Introduction

Swallowing is a neuromuscular process consisting of a complex sequence of events aimed at moving nutrients from the mouth to the stomach passing through pharynx and esophagus [START_REF] Ertekin | Neurophysiology of swallowing[END_REF]. This sensorimotor task includes the voluntary and reflexive activation of several muscles to produce a highly coordinated movement [START_REF] Hamdy | The anatomy and physiology of normal and abnormal swallowing in oropharyngeal dysphagia[END_REF]. There are three swallowing phases: oral, pharyngeal and esophageal. The oral phase involves the voluntary activation of many muscles aimed at the manipulation and propulsion of the bolus. In this phase, the bolus volume and density are factors affecting the degree and timing of muscle activation. The pharyngeal phase includes the reflexive activation of two muscular groups: submental and infrahyoid. The former allows the beginning of the swallow by elevating the hyolaringeal complex in the antero-superior direction, the latter depresses the mandible and the hyoid bone at the end of the swallow [START_REF] Ertekin | Neurophysiology of swallowing[END_REF]. The last esophageal phase is governed by the peristaltic wave pushing the bolus into the stomach [START_REF] Matsuo | Anatomy and Physiology of Feeding and Swallowing : Normal and Abnormal[END_REF].

An objective assessment of the swallowing process is clinically relevant not only to understand the pathophysiology of swallowing impairment (referred to as dysphagia), but also to target its treatment and quantitatively follow its evolution [START_REF] Martino | Incidence , Diagnosis , and Pulmonary Complications[END_REF]. In this regard, the assessment of muscle activation is usually performed through the analysis of left-right symmetry in surface electromyographic (sEMG) activation patterns [START_REF] Zhu | Evaluation of normal swallowing functions by using dynamic highdensity surface electromyography maps[END_REF]. To the best of our knowledge, however, the symmetry indexes are computed without performing any temporal segmentation (i.e. considering both the oral and pharyngeal phases of the swallowing). This provides a global symmetry indication that includes the voluntary activation of the oral phase and may mask shorttime asymmetries associated with the pharyngeal phase. An assessment of EMG asymmetries would require the identification of the swallowing phases. Although the oral and pharyngeal phases are highly interrelated and the distinction between them is often unclear [6], specific events can be used to separate them. This can be performed through the fiberoptic endoscopic examination of swallowing (FEES), an invasive technique that allows to visualize the transit of the bolus in the oropharyngeal cavity. Although this technique is a clinical standard, due to its invasiveness, alternative approaches have been developed in recent years. Zoratto et al. [START_REF] Zoratto | Hyolaryngeal excursion as the physiological source of swallowing accelerometry signals Hyolaryngeal excursion as the physiological source of swallowing accelerometry signals[END_REF], for instance, investigated the possibility of quantifying the hyolaringeal excursion through accelerometry to perform a biomechanical evaluation of swallowing.

In this study we designed a non-invasive measurement framework integrating accelerometric and electromyographic measures to obtain event-related muscular symmetry indexes. The specific aim was the comparison of the accelerometer-based segmentation with that obtained through the clinical standard (FEES) in terms of (i) swallowing onset detection and (ii) muscular activation symmetry during the oropharyngeal phase of swallowing.

Methods

Subjects

Five participants (three males and two female, age range: 31-42 years), with no history of swallowing impairments were recruited. The study was approved by the ethical committee of the Casa di Cura Privata del Policlinico di Milano (Local Ethical Committee: Milano Area 2; Resolution 718_2019; ID 1112). The study was conducted in accordance with the Declaration of Helsinki and written informed consent was obtained from all participants after receiving detailed explanation of the study procedure.

Protocol

Experimental Setup. The proposed experimental setup involved: (i) accelerometry for the temporal segmentation of the swallowing event based on hyolaringeal excursion, (ii) FEES technique to obtain reference values for the comparison with accelerometry results, (iii) sEMG detection for the evaluation of the electrical muscular activity (Figure 1).

Accelerometer recordings. Laryngeal elevation was monitored by an ultracompact linear tri-axis accelerometer with a selected full scale of ±2g (LIS344ALH, STMicroelectronics, Netherland) placed on the skin at the level of the cricothyroid space. Anatomical landmarks (Thyroid and Cricoid cartilages) were used to ensure the correct placement of the accelerometer. Only vibrations along the antero-posterior direction were considered, as suggested by previous studies [START_REF] Zoratto | Hyolaryngeal excursion as the physiological source of swallowing accelerometry signals Hyolaryngeal excursion as the physiological source of swallowing accelerometry signals[END_REF]- [START_REF] Reddy | Measurements of acceleration during videofluorographic evaluation of dysphagic patients[END_REF]. Accelerometer signals were amplified, sampled at 2048Hz and A/D converted with 16-bit resolution through a wireless system (DueBio, OT Bioelettronica, Torino, Italy).

Videoendoscopic recordings. The clinical evaluation of swallowing was performed by introducing a flexible fiber optic, transnasally (Optomic, Madrid, Spain). The correct placement of the fiber optic was ensured by an expert speech pathologist. Videos were acquired synchronously with accelerometer and sEMG recordings with a frame rate of 25fps and stored in a personal computer for further analyses.

sEMG recordings. Muscle activity was recorded using high-density sEMG technique, i.e. with two grids of 32 electrodes (4 rows by 8 columns with an inter-electrode distance (IED) of 10mm -LISiN, Politecnico di Torino, Torino, Italy). Electrodes grids were taped on the skin using a double-sided adhesive tape and placed above the region of interest after an appropriate skin preparation [START_REF] Hermens | Development of recommendations for SEMG sensors and sensor placement procedures[END_REF]. Both submental and infrahyoid muscular activities were recorded. The former muscular group was covered by a grid of electrodes positioned with the smaller symmetry axis lying on the sagittal plane of the subject. The latter was inspected through a custom-made cross-shaped grid of electrodes, specifically designed to allow the positioning of the accelerometer over the skin. Electromyographic signals were detected in monopolar derivation (referred to a reference placed behind the ear), amplified, bandpass filtered (10-500 Hz), sampled at 2048Hz and A/D converted with 16-bit resolution through a miniaturized EMG acquisition system (MEACS, LISiN, Politecnico di Torino, Turin, Italy [START_REF] Cerone | A Modular , Smart , and Wearable System for High Density sEMG Detection[END_REF] ). Experimental Procedure. Subjects were comfortably seated on a chair and were required to keep the same position of the head during the entire protocol. Different swallowing tasks were performed: saliva (dry swallow), 3ml and 10ml of water, 3ml and 10ml of gelled water (Resource Aqua+, Nestlè, Switzerland). Three trials were performed for each task. Before starting the recording, subjects held the bolus in the oral cavity until the instruction to swallow given by the experimenter. An external synchronizing trigger was used to identify the start and the end of the video FEES acquisition on accelerometer and EMG signals.

Data analysis

All recorded data were processed in Matlab (R2019b, The MathWorks Inc., MA, USA). Video recordings from FEES were visually inspected through a frame by frame analysis. The identification of the swallowing onset (tFEES) was performed isolating the time instant relative to the first "white frame", as shown in Figure 3 [START_REF] Nacci | Fiberoptic endoscopic evaluation of swallowing ( fees ): proposal for informed consent[END_REF]. This frame is known as video whiteout and it is produced by the obliteration of the tip of the endoscope due to the pharyngeal restriction [START_REF] Perlman | Dysphagia in Stroke Patients[END_REF].

Figure 3 -FEES frames of interest of a representative subject during a single task (swallowing of 3 ml of water). From left to right: rest condition, retraction of the tongue, epiglottis tilting, whiteout, end of the swallow. The squared red frame refers to the swallowing onset extracted from the video recordings (tFEES).

The effects of the method (accelerometer vs FEES) and of the type of bolus on the swallowing onset estimations were tested separately through a Friedman's ANOVA. The extracted swallowing onsets (tFEES and tACC) were used to compare accelerometry and FEES methodologies. To this end, statistical analyses were carried out on the time differences between tACC and tFEES, for each trial.

Surface EMG signals.

Raw signals were band-pass filtered in the 20-400Hz frequency band (4th order, zero-lag, Butterworth filter). For quantitative analyses, bipolar signals were estimated from the HD-sEMG recordings for both submental and infrahyoid muscles. For each side of the grids (left and right with respect to the two halves) we differentiated two of monopolar signals 2cm apart to obtain one bipolar channel mimicking a conventional sEMG detection [START_REF] Vieira | Spatial variation of compound muscle action potentials across human gastrocnemius medialis[END_REF]. For each side of the grid 8 pairs of electrodes could be used for this purpose, we selected those located in the muscle region with the highest monopolar activity.

The swallowing onsets identified through accelerometer and FEES (tACC and tFEES) were used to segment sEMG signals. Specifically, an epoch of 1.5s after each swallowing onset was considered and the signals of this epoch was used to compute the left/right symmetry index (LRI), according to the following formula of LRI = %1 - LRI of all the trials were averaged separately for each subject. The effect of the segmentation method on the muscular activation symmetry evaluation (LRI) was analyzed using the Friedman's ANOVA.

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Results

Swallowing onset estimation

Recruited subjects completed all the requested tasks without reporting problems or excessive discomfort due to FEES procedures. The quality of the acquired videos was sufficient to clearly identify the whiteout frame in every recording. As regards the accelerometer-based swallowing onset, only the 10 signals out of 75 acquisitions were excluded after the visual cross-checking of automatically detected onsets. Statistical analysis showed no statistically significant effect of the bolus type on the swallowing onset estimation. On the other hand, statistically significant differences (p<0.05) were found between the swallowing onset identified with the two methods (accelerometry and FEES). In fact, the swallowing onset identified through the accelerometer preceded that identified by FEES of 204±192 ms (mean and standard deviation across subjects, bolus type and trials), Figure 4.

Muscular activation symmetry evaluation

Results of symmetry indexes of bipolar EMG signals are reported in Figure 5. Averaged LRI across trials, separately for each subject, were always higher than 85% for both muscular regions under investigation. The mean LRI value across all the participants was of 93±3% (mean±std) for both muscular regions and segmentation methods. Results showed no statistically significant differences (p>0.05) between LRI values computed after segmenting the sEMG signals with the two methodologies: FEES or accelerometry.

Discussion

As compared to the clinical standard (FEES), accelerometry revealed a systematic bias towards an underestimation of the swallowing onset, regardless to the bolus type or volume. It is worthy to note that the time uncertainty, considering the possible delay among recording systems, the video frame rate and the electromechanical delay of anatomical structure movements is about dozens of ms. As a result, the identified time instants could be shifted of this quantity on time axis. Nevertheless, since the LRI values were not statistically different when estimated with the two methodologies (accelerometry and FEES), the observed underestimation is suggested to be not relevant in terms of sEMG-based symmetry estimation. As a result, this findings suggested the use of accelerometer traces as a possible method for the swallowing onset identification as far as measures are aimed at finding symmetry differences in swallowing muscular activation.

Finally, high LRI values were in line with expectations since participants were healthy subjects. Event-related LRI were not compared to global ones (i.e. including portions of the signal before the hyolaringeal excursion) since the standardization of the oral phase of our experimental protocol.

Conclusions

The main goal of this study was to design an experimental setup integrating biomechanical and electrophysiological measures to: (i) provide an alternative, noninvasive method to identify the swallowing onset, (ii) compute event-related symmetry indexes. This approach was sought to improve the global symmetry analysis used in literature.

An experimental protocol aimed at the validation of accelerometer-based swallowing segmentation was carried out. The comparison with the standard FEESbased segmentation showed that accelerometry consistently underestimated the swallowing onset. Nevertheless, this underestimation did not seem to affect the sEMG evaluation outcomes. Indeed, event-related symmetry indexes revealed to be comparable when performed with the two segmentation techniques. Although preliminary, these results suggest that accelerometric measures can provide a possible alternative to the invasive FEES-based segmentation for the extraction of eventrelated symmetry indexes during the oropharyngeal phase of swallowing.

Considering the positive methodological improvements on the assessment of the swallowing function on healthy subjects, the integrated experimental framework proposed in this study is ready for a validation on a larger sample population.

Figure 1 -

 1 Figure 1 -Schematic representation of the proposed experimental setup: accelerometry and endoscopy for the mechanical evaluation of swallowing and HD-sEMG systems for the electrophysiological evaluation of submental and infrahyoid muscles.

  Accelerometry and Videoendoscopic data. The swallowing onset based on hyolaringeal excursion was obtained through the analysis of accelerometer signals as shown in Figure2. After low pass filtering the raw signal at 4Hz (4 th order, zero-lag Butterworth filter), a threshold was set at three standard deviations of the baseline estimated over a 1.5s epoch in the rest phase before the swallowing. The accelerometer-based swallowing onset (tACC) was identified as the time instant corresponding to the first sample of the filtered signal crossing the threshold. A visual inspection was necessary to verify the correctness of the automatically-identified time onsets and to exclude spurious onsets due to noise or artifact-related threshold crossing.

Figure 2 -

 2 Figure 2 -Accelerometer signal of a representative subject performing a single task (swallowing of 3ml of water). The dashed green line shows the swallowing onset (tACC) identified using an amplitude threshold of 3 standard deviations of the baseline (black horizontal line).

Figure 4 -

 4 Figure 4 -Time differences between the accelerometer-based and the FEES-based swallowing onset. Positive and negative values refer respectively to late and beforehand tACC with respect to tFEES (t = 0ms). Mean and standard deviation refer to al trials and bolus for all subjects (* p<0.05).

Figure 5 -

 5 Figure 5 -Left/Right Indexes (LRI) computed on bipolar sEMG signals in a 1.5s epoch starting from swallowing onset estimations based on FEES (red traces) and accelerometer (green traces). Data from submental (left panel) and infrahyoid muscles (right panel) are reported.


Marco Marini

Mauro Viganò

Massimo Corbo

Marina Zettin

Gloria Simoncini

Bruno Fattori

Clelia D'anna

Massimiliano Donati

Luca Fanucci

IDEA: An Italian Dysarthric Speech Database

Keywords: speech recognition, dysarthria, database, kaldi

niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

INTRODUCTION

The evolution of automatic speech recognition (ASR) systems allows us to use smart devices just through our voice. It could be very useful for people with motor impairments or severe problem to handle smart devices. But what happens when people have impaired speech? People affected by motor speech disorders have difficulties using ASR systems according to the level of disorder [START_REF] Ballati | hey siri, do you understand me?": Virtual assistants and dysarthria[END_REF][START_REF] Ballati | Assessing virtual assistant capabilities with italian dysarthric speech[END_REF][START_REF] Mulfari | Towards a deep learning based asr system for users with dysarthria[END_REF][START_REF] Mulfari | Machine learning in assistive technology: a solution for people with dysarthria[END_REF]. One of these types of disorder is the dysarthria.

Dysarthria is a neurological acquired motor speech disorder [START_REF] Malcolm R Mcneil | Clinical management of sensorimotor speech disorders[END_REF] that can impair all processes involved in speech pro-duction such as respiration, phonation, articulation, resonance and prosody. These manifestations, and imprecise articulation in particular, often result in reduced speech intelligibility, which could negatively impact on the quality of life and the social participation of the speakers [START_REF] Mary | Does speech and language therapy work?: A review of the literature[END_REF].

To improve the performance of ASR systems for dysarthric speakers, is fundamental to have enough data to analyse and use in training process. For this reason, during the past years several scientists have developed dysarthric speech databases.

The Whitaker database [START_REF] Jr Deller | The whitaker database of dysarthric (cerebral palsy) speech[END_REF] is a collection of 19275 isolated-word utterances spoken by 6 persons whose speech spans a broad spectrum of dysarthria due to cerebral palsy. It was implemented in 1993.

Nemours database [START_REF] Menendez-Pidal | The nemours database of dysarthric speech[END_REF] was developed later and contains which contains 814 short nonsense sentences and 74 sentences spoken by each of 11 male speakers with varying degrees of dysarthria.

Universal Access speech database (UASpeech) [START_REF] Kim | Dysarthric speech database for universal access research[END_REF] was delivered online in 2008 and consist of dysarthric speech recordings produced by 19 speakers with cerebral palsy. Speech materials consist of 765 isolated words per speaker.

All these databases contain single word or short sentence recordings of dysarthric speakers. Some of those also have video information that enrichs the database and helps the researchers. TORGO database [START_REF] Rudzicz | The torgo database of acoustic and articulatory speech from speakers with dysarthria[END_REF] store also articulatory data in addition to recordings. Articulatory data is obtained by electromagnetic articulography, which allows the measurement of the tongue and other articulators during speech, and by 3D reconstruction from binocular video sequences.

It is quite clear that several efforts have been made to collect English dysarthric voices for scientific research purposes. In the best of our knowledge, an Italian dysarthric speech database does not exist until today. It is very important to have a database of a specific language because the techniques discovered to improve speech recognition for English speakers may not be good for Italian ones. So, an Italian dysarthric speech database is useful to test the goodness of ASR system for English dysarthric speakers and to make progress in this

D R A F T topic but in multi-language context.
With the purpose to develop IDEA: the first Italian dysarthric database, we start a collaboration with 3 health facilities: Azienda Ospedaliero-Universitaria Pisana (AOUP) [START_REF]Azienda ospedaliera universitaria pisana[END_REF], Centro Puzzle di Torino [START_REF]Centro puzzle di torino[END_REF] and Casa di Cura Privata del Policlinico di Milano [START_REF]Casa di cura privata del policlinico di milano[END_REF]. All these centres are specialized in diagnosis and rehabilitation of speech and language pathologies and host several dysarthric speakers for daily therapy and tests, or for long time hospitalization.

Specially, we have to distinguish our partners in two categories: Long Term Hospitalization (LTH) facilities and Daily Medical Examination (DME). The former provides for treatment of patients for several weeks or also months, so in this kind of facilities there is the possibility to record the same patient's voice for long time and in different days.

In daily medical examination facilities, the specialists have just few hours to evaluate the patient health. In this context there is not the possibility to record a big amount of data from each patient as in the previous case.

Among our partners, Centro Puzzle and Casa di Cura Privata del Policlinico are LTH facilities, while AOUP is DME facility.

Unlike the other databases, IDEA also has a lot of medical information about speakers. This could be very crucial to investigate medical aspects in speech analysis (e.g. dysarthric assessment).

IDEA database is available to anyone willing to use the database for scientific purposes; to access IDEA database is just required to send an email to the authors.

METHOD

Equipment

All the recording procedures have been done in a quiet and free of distractors room inside the facility where the patient has been hosted and respecting all the directives imposed by an ethics commission. The equipment required to start a recording procedure is composed by a computer with screen or a laptop and a condenser microphone [START_REF]GoMic portable usb condenser microphone[END_REF]. The University of Pisa provided the same microphones to each partner, and a specific software for recording named RECORDIA.

RECORDIA

RECORDIA is a software based on Java and has two main functionalities: patient characterization and recording procedure. We will describe in details these functionalities in the following sections.

Patient characterization

The user can add a new patient through an Italian Graphical User Interface (GUI) and in order to do that, it is required to insert personal information (name, surname, gender and age) and medical details. In clinic details the user can choose among 9 pathologies. In case the patient does not have any of those pathologies, the user can select "Other" (OTH) as option and insert some information in a "Note" field.

For each pathology, the user can insert some additional information like date of diagnosis or date of first symptoms, if available. Furthermore, there is the possibility to compile a specific pathology assessment scale according to the pathology. The pairs pathology-scale are showed below:

• Amyotrophic Sclerosis Lateral (ASL) -ASL functional rating scale [START_REF] Young | Korean version of amyotrophic lateral sclerosis functional rating scale-revised: a pilot study on the reliability and validity[END_REF] • Ataxia (ATX) -Brief Ataxia rating scale [START_REF] Camargos | Brief ataxia rating scale: a reliable tool to rate ataxia in a short timeframe[END_REF] • Huntington's Chorea (HC) -Unified Huntington's disease rating scale [START_REF] Siesling | Unified huntington's disease rating scale: a follow up[END_REF] • Multiple Sclerosis (MS) -Kurtzke Expanded Disability Status Scale [START_REF] Donald E Goodkin | Inter-and intrarater scoring agreement using grades 1.0 to 3.5 of the kurtzke expanded disability status scale (edss)[END_REF] • Myotonic Dystrophy (MD) -Muscular Impairment Rating Scale [START_REF] Mathieu | Assessment of a disease-specific muscular impairment rating scale in myotonic dystrophy[END_REF] • Neuropathy (NEU) -Guillain-Barré Syndrome Disability Scale [START_REF] Fokke | Diagnosis of guillain-barré syndrome and validation of brighton criteria[END_REF] • Parkinson Disease (PD) -Unified Parkinson's Disease Rating Scale [START_REF]Movement Disorder Society Task Force on Rating Scales for Parkinson's Disease, "The unified parkinson's disease rating scale (updrs): status and recommendations[END_REF] • Stroke (STR) -National Institutes of Health Stroke Scale [START_REF] Lyden | Underlying structure of the national institutes of health stroke scale: results of a factor analysis[END_REF] • Traumatic Brain Injury (TBI) -Levels of Cognitive Functioning [START_REF] Gouvier | Reliability and validity of the disability rating scale and the levels of cognitive functioning scale in monitoring recovery from severe head injury[END_REF] In addition to clinical information, the user can classify the type of dysarthria with a specific taxonomy proposed by Duffy [START_REF] Joseph R Duffy | Motor speech disorders-e-book: Substrates, differential diagnosis, and management[END_REF], evaluate the level of impairment with the Robertson Dysarthria Profile [START_REF] Defazio | Assessment of voice and speech symptoms in early parkinson's disease by the robertson dysarthria profile[END_REF]. The user also can fill out a therapy outcome measure scale [START_REF] Enderby | Therapy outcome measures for rehabilitation professionals: speech and language therapy, physiotherapy, occupational therapy[END_REF] and measure the quality of life in the dysarthric speaker [START_REF] Piacentini | Reliability and validity of an instrument to measure quality of life in the dysarthric speaker[END_REF].

All this information about the patient are editable over time, so it is possible to update or insert missing details when available.

Registration procedure

As explained in the previous chapters, the registration procedure is supervised by a user, typically a speech-language pathologist or a specialist, who interacts with the GUI and checks that the whole process goes properly.

In the registration procedure, RECORDIA software shows at screen a sequence of words that speaker has to repeat aloud. When a new word appears on the screen, the speaker has a fixed amount of time to pronounce it. This amount of time (2, 3 or 5 seconds) is chosen by the user at the beginning according to the ability of the speaker. If the D R A F T speaker gets trouble to do it, the user can try again. If the recording was successful, the user can move to the next one automatically proposed by RECORDIA. The next word is chosen so that the same word does not appear very close in time and as much random as possible. Each word recorded is stored in a single wave file. More repetitions of the same word have different wave files. The total amount of words is 211 chosen by doctors to cover the pronunciation of all Italian phonemes.

As explained in the Introduction, our partners have two different manners to deal with patients. Some have the chance to have the patients at their disposal for long period of time (LTH facilities). The others have just a bunch of minutes to record the patients' voices (DME clinics). For this reason, RECORDIA offers two levels of recording procedures.

The Level 1 is suited for DME clinics because it requires about 15 minutes to complete the whole procedure. The aim of this level is to acquire a good number of recordings from a speaker despite a limited time at disposal. With good number of recordings, we mean 3 repetitions for 45 words. So, as soon as the speaker has recorded all the 135 wave files, the recording session ends. It is possible to stop the session early for any reason and restart it later from the same point. The Figure 1 shows the list of 45 words that are composed by a prefixed subgroup of 44 words of 211 and the word "occhi". They are chosen in order to have at least one pronunciation of each phoneme. In Figure 2 is shown a screenshot of RECORDIA GUI during Level 1 recording procedure of word "troppo".

The Level 2 uses all 211 words (listed in Figure 3) and does not have any stop criteria as Level 1. So, the user can record as much as words he wants and stop the session at any time. This is used by LTH facilities because they have a lot of time at disposal to record dysarthric voices as much as possible.

In the first level the amount of data recorded is fixed while in the second level is unlimited. For both the Levels, the recordings are stored in the local PC and then sent to the University of Pisa by the user. Furthermore, they allow the user to collect a comprehensive Italian phoneme inventory, with at least a sample of all the 30 Italian phonemes [START_REF] Tresoldi | Normative and validation data of an articulation test for italianspeaking children[END_REF]. The Table 1 shows all the phonemes involved in IDEA database divided by classes. In this table it is also possible see the distribution of occurrence of different phonemes over all 211 words. The geminates phonemes are consonants that have different sound duration. Furthermore, both levels include several bi-consonant and tri-consonant clusters but a full examination of clusters was not included in order to limit the number of words and to keep the procedure feasible with impaired speakers.

RECORDIA also gives the possibility to change the contrast of the word showed on screen. This feature is useful for people with sight issues.

DESCRIPTION OF DATABASE

IDEA has 16794 total recordings distributed among 45 speakers, 25 males and 20 females. The total amount of time recorded is about 13.72 hours of which about 35% is speech (about 4.1 hours) and 65% is non speech (9.62 hours). For DME speakers, we have recorded 45 words for 3 times in the same day for a total of 135 wave files for each dysarthric speaker. For LTH speakers, our aim was to record at least 3 times all 211 words in different days for a total of 633 wave files for a single speaker. Since a dysarthric speaker typically is affected by other pathologies, a recording process could be very stressful and tiring. Hence, some speakers interrupted the recording procedure, so for them some recordings are missing. Each wave file is encoded in the linear PCM format at 16 bit and 16kHz sample rate. The Table 2 shows some statistics for each speaker, specially the type of facility, gender, amount of recordings, percentage of recordings done respect to the final aim and pathology. Thus, 21 speakers (46% of total) have complete the entire recording procedure, 8 speakers (17% of total) have recorded less than 50% of the pre-established recordings and 16 speakers have completed the procedure between 50% and 100%.

The Figure 4 shows the distribution of pathologies in the IDEA database. This information can be inferred by the last column of Table 2.

Annotation

We have analysed all the data in order to detect and annotate any errors or corruptions. This annotation could be useful to organise the data efficiently during ASR training. With this purpose, we decided to classify wave files in 4 categories:

• Empty: the file is empty or not playable (due to some RECORDIA failure);

• Not usable: the wave file does not contain speech but just noise, or the noise is too loud to understand speech;

• Without notes: the wave file is correctly recorded and the speech is pronounced in absence of noise;

• With notes: the wave file has some issues that should be tacked into account when we deal with this file.

Each wave file has a TextGrid file with the same name. A TextGrid file is a Praat file [START_REF] Boersma | Praat: doing phonetics by computer[END_REF], which contains some information about wave file like duration and tiers. For instance, for a wave file without notes the associated TextGrid file will describe what are the start and stop points of speech inside the recordings and what exactly is pronounced by the speaker within these boundaries.

If the wave file is classified as with notes, in the TextGrid file will appear one more tier which describe it. The possible issues are:

• Truncated: the speaker speech is truncated because is too close to the beginning or end of record;

• Substitution: the speaker pronounced another word rather than the one proposed on screen;

• Repetition: the speaker pronounced more times the same word within wave file;

• Corrupted: wave file present some microphone saturation or glitch;

• Background noise: the wave file presents some background noise, like coughing, but the speech is still understandable; D R A F T Fig. 3. Level 2 list of words.

• Word split: the speaker makes a pause in the middle of the word or tend to pronounce the word in syllabic way;

• General notes: is used to annotate some generic issue like if the speaker pronounces the word longer than usual (e.g. "donnnna" instead of "donna").

The 70.8% of records are without notes, so it means that they are usable as they are. The 22.8% are with notes, so they need to be used with more attention and just the 6,4% is unusable (5.1% are empty and 1.3% are not usable).

BASELINE ASR EXPERIMENTS

Experimental setup

In the first version of this database (IDEA 1.0), we decided to insert all the speakers who have enough data usable as it is. For this reason, we considered only the recordings without notes for all speakers. Thus, we tried to split these recordings in train and test sets with the following approach: if a word

Results

One of the purposes of developing IDEA database is to create an ASR system for Italian dysarthric speakers. So, we tried to run a Kaldi [START_REF] Povey | The kaldi speech recognition toolkit[END_REF] recipe baseline for UA-Speech corpus used in [START_REF] Xiong | Deep learning of articulatory-based representations and applications for improving dysarthric speech recognition[END_REF], upon IDEA 1.0 database. We decided to use this recipe because UA-Speech database contains single word recordings as IDEA database. Of course, the data preparation of the recipe it is adjusted for IDEA 1.0 database, but the feature extraction and acoustic modelling parts are the same. We evaluated the Word Error Rate (WER) for two models: Gaussian Mixture Model (GMM) based and Deep Neural Network (DNN) based, both combined with Hidden Markov Model (HMM). Table 3 shows that the GMM-HMM ASR system got wrong 254 recordings out of 2161, which means 11.75% of WER. On the other hand, the DNN-HMM ASR system got wrong 324 recordings which means 14.99% of WER. The results for each speaker are shown in Figure 5. The results show that GMM-HMM perform better than DNN-HMM. This differs form results obtained in [START_REF] Xiong | Deep learning of articulatory-based representations and applications for improving dysarthric speech recognition[END_REF] but it could be explained by the limited amount of data for training. Indeed, IDEA 1.0 has 7.84h for training and 1.89h for testing from 45 speakers, while [START_REF] Xiong | Deep learning of articulatory-based representations and applications for improving dysarthric speech recognition[END_REF] uses 23.1h for training and 21.7h for test from 15 speakers. So, probably the data is not enough to train all the neural network weights properly.

Despite the WER is very high for some speaker, as shown in Figure 5, the overall WER is quite low for both types of models. This is because the distribution of recordings in each speaker's test sets is not balanced. For instance, from GMM-HMM model point of view the speaker 201 has 100% of WER but she also has just 4 recordings in test set, so it does not have a big impact on final WER.

CONCLUSION

In this paper we have describe IDEA: the first Italian dysarthric speech database. It contains the voices of 45 speakers for a total of 16794 recordings for about 13 hours and 43 minutes. Single word recording was performed for 211 words. Each record has a TextGrid file associated that defines the boundaries of speech in the wave file and adds other important information. Moreover, IDEA store specific medical information about speaker like type of pathologies (there are 9 different options), a specific assessment scale for each type of pathology and other clinical test. We believe that this corpus provides a significant resource for develop assistive technologies since it contains medical and technical information for a good number of users. Some preliminary test has been done to develop an ASR system using Kaldi recipe. This first experiment has been done using about 67% of all IDEA material and shows that ASR systems based on GMM model perform better than DNN model for this amount of data. In the future work we want to use the whole database for training ASR systems and to investigate how to involve clinical information in training process. We are continuing to record other volunteers to further expand our database. IDEA is available free, people interested on IDEA have to contact the author of this paper, who will give the possibility to download it.

ACKNOWLEDGMENTS

The authors thank the volunteers for their participation. Much data processing was done with the help of Heidi Christensen, Phil Green and Mauro Nicolao of University of Sheffield (UK).

Fig. 1 .

 1 Fig. 1. Level 1 list of words.

Fig. 2 .

 2 Fig. 2. Screenshot of RECORDIA GUI during Level 1 recording procedure.

Fig. 4 .

 4 Fig. 4. Percentage of pathologies present in IDEA database.

Fig. 5 .

 5 Fig. 5. Baseline results: WER of ASR systems for each speaker.

Table 1 .

 1 Phonemes involved in IDEA database divided by classes and with: relative symbol, IPA symbol and distribution of occurrences over 211 words.

		Class	Symbol IPA Occurrences
			p	p	34
		plosives (6)	b t d k	b t d k	17 52 35 39
			g	g	16
	D	fricative (5) affricates (4)	f v s z S tS ts dz	f v s z S tS ts dz	24 21 49 13 2 15 5 2
	R	nasals (3) liquids (3)	dZ m n J l r LL	dZ m n ñ l r L	11 38 77 1 53 95 4
			a	a	151
	A	vowels (7)	e E i o O u JJ SS	e E i o O u ñ: S:	96 38 91 132 26 27 3 1
			bb	b:	3
	F T	geminates (19)	dd ddZ ddz ff gg kk ll mm nn pp rr ss tt ttS tts	d: ddZ: dz: f: g: k: l: m: n: p: r: s: t: tS: ts:	1 2 1 4 1 6 3 1 5 1 4 9 10 2 4
			vv	v:	1
		approximants (2)	j w	j w	18 10

Table 2 .

 2 Statistics about speakers: ID, gender, facility type (see Introduction for detail), number of records, percentage of completion of total recordings and pathology.

	ID Gend Facility # REC Complete Pathology
	201	F	DME	135	100.00%	OT H
	202	F	DME	39	28.89%	HC
	203	F	DME	135	100.00%	ST R
	204	F	DME	41	30.37%	P D
	205	M	DME	135	100.00%	AT X
	206	M	DME	135	100.00%	ASL
	207 208 209 210 211 212 213 D	M F F M F F F	DME DME DME DME DME DME DME	135 135 135 135 135 135 135	100.00% 100.00% 100.00% 100.00% 100.00% 100.00% 100.00%	ST R P D P D MD ASL ASL ASL
	214	M	DME	135	100.00%	ASL
	215 216 301 302 303 304 305 R	M F M M M F F	DME DME LTH LTH LTH LTH LTH	135 135 630 630 630 14 633	100.00% 100.00% 99.53% 99.53% 99.53% 2.21% 100.00%	P D ST R P D ST R ST R OT H T BI
	306	M	LTH	632	99.84%	OT H
	307	F	LTH	630	99.53%	T BI
	308 309 310 311 312 313 314 A	M M F M F M M	LTH LTH LTH LTH LTH LTH LTH	634 178 630 634 631 576 630	100.16% 28.12% 99.53% 100.16% 99.68% 91.00% 99.53%	T BI T BI T BI T BI T BI ST R OT H
	315	M	LTH	631	99.68%	T BI
	316 317 318 319 320 321 322 F	F F M M F M M	LTH LTH LTH LTH LTH LTH LTH	630 445 217 420 634 633 637	99.53% 70.30% 34.28% 66.35% 100.16% 100.00% 100.63%	T BI MS OT H T BI T BI OT H OT H
	323	M	LTH	635	100.32%	T BI
	401	M	LTH	254	40.13%	MS
	402 403 404 405 406 T	M F F M M	LTH LTH LTH LTH LTH	589 151 552 534 150	93.05% 23.85% 87.20% 84.36% 23.70%	ASL P D OT H ST R ASL
	has 3 or more records, 1/3 goes to test and 2/3 go to train.
	For those words that have less than 3 records, all are used
	for train. Obviously, for those speakers who have not enough

Table 3 .

 3 Baseline results with numbers of recordings in train and test sets, number of errors and WER for each ASR system.

		Train	Test	Errors	WER
	GMM 9111 (7.84h) 2161 (1.89h) 254	11.75%
	DNN	9111 (7.84h) 2161 (1.89h) 324	14.99%


THE BIRTH Jules Janssen

J.-M Malherbe
email: jean-marie.malherbe@obspm.fr

THE FOUNDATION OF MEUDON OBSERVATORY AND THE MONT BLANC ADVENTURE (1875-1895)

Keywords: Janssen, Sun, physical astronomy, imagery, spectroscopy, solar spectrum, observatories, Meudon, Mont Blanc

Jules Janssen is a well known astronomer. He introduced in 1868 the spectroscopic technique to observe permanently solar prominences. He also invented innovative methods in imagery and photography. This paper focuses on the period going from the foundation of Meudon Observatory, associated to the birth of astrophysics and solar physics in France, to the Mont Blanc saga, with the creation of the observatory and the three scientific ascents of Janssen to the top of the mountain. As most documents are written in French, this fantastic and unbelievable adventure for the end of the XIX th century, deserves to be related in English, and reveals the long term vision of Janssen's astronomy. It involved specialized instruments, located in well chosen places where atmospheric troubles are minimized. High altitude observations, at Mont Blanc (or from balloons also experienced by Janssen), prefigure space astronomy which will develop much later. Hence, Janssen appears as a precursor of modern astronomy, well ahead of his time.

INTRODUCTION

Jules Janssen was born in 1824 (Figure 1). He was attracted late by scientific studies (mathematics, physics) and became "docteur ès sciences" in 1860. His thesis did not concern astronomy, but optical properties of the eye's medium. The works of Gustav Kirchhoff (1824Kirchhoff ( -1887) ) and Robert Bunsen (1811Bunsen ( -1899) ) in the field of spectroscopy, explaining the existence of lines discovered by Joseph von Fraunhofer (1787-1826) in the solar spectrum, had a strong influence on Janssen. He soon understood that spectroscopy will permit the determination of the chemical composition of the Sun and stars. Janssen started spectroscopic observations in 1862, designed direct vision spectroscopes based on Giovanni Amici (1786-1863) prisms, and became an incredible and tireless globe-trotter [START_REF] Launay | The Astronomer Jules Janssen: A Globetrotter of Celestial Physics[END_REF] to catch solar eclipses. He observed the solar spectrum in many high altitude stations, in order to separate the solar and telluric lines (atmospheric lines, such as dioxygen O2 or water vapor H2O, weaken with altitude). The most fruitful expedition was probably the long eclipse of 18 August 1868 in India (6 min 47 s duration according to NASA) reported by [START_REF] Launay | Observations of the total solar eclipse of 18 August 1868 carried out by Jules Janssen at Guntoor, India[END_REF]. Janssen observed solar prominences around the solar disk masked by the Moon. These features are cold hydrogen condensations in the hot solar corona, revealed by the red light of the Hα line emission (656.3 nm wavelength). Prominences are not directly visible outside eclipses due to the brightness of the disk. After the phenomenon, Janssen positioned his spectroscope on the prominence region and saw that Hα emission was present in the spectrum ! This major discovery, which was also made independently at the same time by Norman Lockyer , permitted hereafter observations of solar prominences at any time in Hα or other chromospheric lines. In 1869, Janssen formulated the principle of the spectro-helioscope to form monochromatic images of solar structures. With a spectroscope rotating at high speed around its optical axis, the entrance slit scans a 2D field of view, and a second slit selects, on exit, a broad line in the solar spectrum. Hence, the retinal persistence provides monochromatic images of the Sun in the selected waveband. This discovery is at the base of the photographic spectro-heliograph invented independently in 1892 by Henri Deslandres (1853Deslandres ( -1848) ) at Paris andGeorge Hale (1868-1938) at Chicago. Modern spectro-heliographs are still in use today (in Meudon or Coïmbra). During the 1868 eclipse in India, Janssen also noticed a yellow line in the spectrum of prominences, but he was not interested in. Norman Pogson (1829Pogson ( -1891) ) distinguished a new line just besides the well known chromospheric D1/D2 sodium doublet [START_REF] Nath | Norman Robert Pogson and observations of the total solar eclipse of 1868 from Masulipatam, India[END_REF], and Lockyer concluded that it was a new chemical element, helium, revealed by the yellow D3 line (587.6 nm wavelength). Helium will be found in minerals three decades later. In 1870, Paris was besieged by the Prussian army, and Janssen used a balloon flight to escape the enemy and attend another eclipse in Algeria; this uncommon event developed his fondness for aeronautics. In 1871, back in India for a new eclipse, Janssen observed the green line at 530.3 nm (later called coronium), previously discovered by Pogson in 1868 [START_REF] Nath | Norman Robert Pogson and observations of the total solar eclipse of 1868 from Masulipatam, India[END_REF], although usually ascribed to Charles Young during the 1869 eclipse in America. At that time, the atomic structure was unknown, so that coronium will only be identified 60 years later (in the 1930s by W. Gotrian and B. Edlén) as a forbidden line of the highly ionised FeXIV (13 lost electrons), indicating that the corona is 200 times hotter than the solar surface, a great advance in solar physics ! Janssen was elected in 1873 at the French Academy of Sciences. He was the inventor of the photographic revolver [START_REF] Launay | Jules Janssen's "Revolver photographique" and its British derivative, "The Janssen slide[END_REF] to record the Venus transit observed in Japan in 1874, and more generally to produce motion pictures of dynamic events. The revolver may be considered as a remote ancestor of the cinematograph. Let us now come to the foundation of Meudon in 1875 and to the birth of physical astronomy in France.

Figure 1: Famous astronomer Jules Janssen (1824Janssen ( -1907) ) founded Meudon Observatory in 1875 and started the Mont Blanc adventure in 1888 to study di-oxygen lines of the solar spectrum (courtesy Paris Observatory).

MEUDON AND THE BIRTH OF PHYSICAL ASTRONOMY AND SOLAR PHYSICS

Meudon is an old royal domain. The first chateau (XVI th century) was burnt down in 1795 and demolished in 1806. The new chateau, built under King Louis XIV in 1706 for his son, the "Grand Dauphin", was occupied by the Prussian army in 1870, which installed gun batteries there in order to shoot over Paris; this chateau was also partly burnt during the war (Figure 2). At this moment, Janssen was convinced that the alliance between emerging techniques, such as photography and spectroscopy, was going to launch a new research field. For that purpose, he was looking for a place to create a large observatory, including spectroscopic laboratories, in a good and large site outside Paris city, in order to develop this new field named "physical astronomy". This science was dedicated to the study of the physical and chemical nature of celestial objects (contrarily to Paris observatory traditionally involved in celestial mechanics and astrometry). The word "astrophysics" was introduced later. This movement occurred also in many countries, and was symbolized by the creation of the Astrophysical Journal (1890) by G. Hale in the USA, and Nature (1869) by N. Lockyer in Great Britain. In France, several scientists, such as Alfred Cornu (1841-1902), Georges Rayet (1839-1906) and others (Le [START_REF] Gars | Cornu : trois parcours exemplaires dans la construction de l'astronomie physique en France (1860-1890)[END_REF][START_REF] Gars | L'émergence de l'astronomie physique en France[END_REF] were implied. In England, previous works of Carl Gauss, Michael Faraday, Jean-Baptiste Biot, André-Marie Ampère were unified by James Maxwell (1831-1879), who produced the theory of electric and magnetic fields, including electromagnetic waves, which had a crucial importance in the development of astrophysics.

Janssen's personal activities, in solar imagery and spectroscopy, are in France at the origin of solar physics. For the new observatory, the government proposed two domains, and Janssen chose Meudon. This place was proposed in 1875 by Mac Mahon and Meudon was definitely affected in 1879 by Jules Grévy, both successive presidents of the French Republic. However, Janssen was authorized to install his instruments as early as 1876. The domain was ruined and occupied by soldiers, so that Janssen began observations of the Sun with temporary huts and instruments used for the Venus transit previously observed in Japan (next section). Meanwhile, an architect was appointed to transform the chateau and prepare the construction of the large 18.5 m diameter dome (Figure 2). The financial support of the government arrived after 1880, so that it took 10-15 years to complete this major project. The transformations and repairs of the chateau were completed in 1893 [START_REF] Janssen | Annales de l'Observatoire d'astronomie physique de Paris sis Parc de Meudon[END_REF][START_REF] Janssen | Mémoires et observations. Note sur les instruments de l'observatoire de Meudon[END_REF]. The 18.5 m iron dome (replaced by copper in 1924) was manufactured by the Cail company (Figure 3). It covers a double refractor, of 16 m focal length, well adapted to observations of small angular diameter objects, such as planets, nebulae, double stars or stellar clusters. The large size of lenses provided excellent resolving power. The optics was made by the Henry brothers of Paris observatory (Paul Henry, 1848-1905, and Prosper Henry, 1849[START_REF] Janssen | Atlas de photographies solaires[END_REF]. The cross section is rectangular (90 cm x 170 cm), because two refractors are superimposed, one of 83 cm diameter (f = 16.10 m) for visual observations and the second of 62 cm (f = 15.90 m) for photographic plates (it was optimized for the blue part of the spectrum). The mechanical mount was a masterpiece of Paul Gautier (1842Gautier ( -1909)). This beautiful instrument was renovated in the sixties [START_REF] Muller | La rénovation de la grande coupole de Meudon[END_REF].

Figure 3: The Meudon chateau is unusual in that it is built across a change in ground level, which explains why Figure 2 shows the telescope part beginning one storey up, whereas the above view shows it beginning at ground level. The large 18.5 m dome was built in 1893 by the Cail company. The mass of the dome is about 100 tons. The double refractor has a visual objective (83 cm diameter) and a photographic objective (62 cm), 16 m focal length, polished by brothers Henry of Paris observatory. The equatorial mount is of German type and was manufactured by the Gautier enterprise. This is the largest refractor in Europe (after [START_REF] Janssen | Annales de l'Observatoire d'astronomie physique de Paris sis Parc de Meudon[END_REF].

The second innovative instrument (Figure 4) was a large 100 cm diameter Newtonian telescope, with short focal length (3 m), so that it was highly luminous (F/3). It was located in a small dome (7.5 m diameter), also built by the Cail company, not far from the large dome of the double refractor. Here again, the mirror was polished by the Henry brothers, and the mechanics was manufactured by P. Gautier. At this time, Meudon instruments were among the most modern in the world. The double refractor is still the largest in Europe, but it is no more in activity. The 100 cm telescope is still used for teaching purposes but the optical combination was changed into a Cassegrain one in 1970, which increased the equivalent focal length to 22 m. This powerful first astronomical equipment was accompanied a variety of laboratories, arranged in the old horse stables, allowing the production of spectra of known gases under various pressures, in order to compare them to celestial ones. A second 7.5 m dome was also constructed by Cail for the refractor dedicated to solar photography (next section).

THE SOLAR PHOTOGRAPHIC ATLAS (1876-1903)

Janssen founded Meudon to develop celestial photography and spectroscopy. The scientific activities, before the construction of the large instruments which required nearly two decades, started with regular observations of the solar surface continuing until the publication of the "Atlas de photographies solaires" by [START_REF] Janssen | Atlas de photographies solaires[END_REF]. This atlas is a selection of what Janssen thought to be the best pictures of the photosphere. Thirty images were chosen among of the 6000 photographic plates obtained from 1876 to 1903. Most correspond to a solar diameter of 30 cm. This big collection was equivalent, in volume (but not in quality), to the lunar atlas of Henri Puiseux and Maurice Loewy, performed between 1893 and 1910 with the "grand équatorial coudé" at Paris observatory (6000 plates also). Unfortunately, all the solar plates but seven were destroyed in the seventies [START_REF] Launay | Jules Janssen et la photographie[END_REF][START_REF] Launay | The Astronomer Jules Janssen: A Globetrotter of Celestial Physics[END_REF]. The photographs of the solar surface were obtained with an excellent refractor (Figure 5, left, and Figure 6) built by several experts [START_REF] Le Cocguen | La lunette de photographie solaire Janssen[END_REF]. The 13.5 cm diameter objective (2.2 m focal length) is due to Adam Prazmowski (1821-1885) and was optimized for the blue-violet part of the spectrum. The mechanical assembly was built by Wilhelm Eichens (1818-1884). The Prazmowski refractor was initially installed on a rolling support made of wood (Figure 6), which complicated the precise orientation of solar images. An equatorial mount was designed later by Eichens for the second 7.5 m dome of the observatory. Photographic plates were produced by the Meudon laboratory with the maximum sensitivity around the G Fraunhofer line (430 nm wavelength). It was a long process to prepare the wet-collodion with iodide and bromide of ammonium, cadmium, or potassium (about 25 g dissolved in a mixture of one litre of alcohol and sulfuric ether). We know today that the contrast of the solar granulation increases towards short wavelengths and is best in the G band. It is composed of many lines of the CH molecule, unresolved in Janssen's epoch, and bright points appear as magnetic proxies of thin concentrated flux tubes. Observations in the G band were revisited in the seventies with large telescopes and narrow interference bandpass filters (0.8 nm); this waveband has become a universal standard in imagery of the solar surface.

Figure 6: The solar refractor used by Janssen in Meudon park. The 135 mm objective was built by A. Prazmowski, the mechanics by W. Eichens and the wood enclosure by Bigot. Special shutters, first made by Prazmowski, and later by Gautier, were designed for exposure times as short as 0.2 milliseconds. The primary image (2 cm) was magnified to 30 cm and formed on wet-collodion plates sensitive to blue light (courtesy Paris Observatory).

Images with the Prazmowski doublet (solar disk of 2 cm diameter) were excellent and optically magnified by an ocular to have a final diameter of 30 cm, and occasionally 50 cm or more according to the seeing. The exposure time was critical to freeze the atmospheric turbulence, which evolves fast (1000 Hz), in order to get high resolution pictures. For that purpose, Janssen imagined a specific shutter design, the "photographic trap" (this is the ancestor of curtain shutters; however, Hippolyte Fizeau and Léon Foucault used similar mechanisms for their 1840s daguerreotypes of the Sun). It is made of a high speed translating slit of adjustable width, which allows exposure times as short as 1/6000 second ! The first traps were made by Prazmowski, but they generated some vibrations. Improved traps were later produced by Gautier. Figure 7 shows an outstanding image of the solar granulation (1877), published in the solar atlas, obtained with the Prazmowski refractor, and highly magnified. It would be impossible today to reach such a quality, because of the seeing and environmental degradation caused by the growth of Paris city. An exceptional picture of a sunspot (1885), with moderate magnification and also published in the atlas, is displayed in Figure 8. Finally, an example of the full Sun ( 1893), obtained at the primary focus of the refractor (without any magnification), is presented in Figure 9 together with the "photographic trap" located in the image plane.

Figure 7: The solar granulation observed by Janssen on 10 October 1877 with special photographic emulsions, prepared in the laboratory and sensitive to the blue part of the solar spectrum, around the Fraunhofer G line at 430 nm wavelength. Granules are small convective cells of 1000 km (1.5 arc second) covering the solar photosphere. The picture quality is not homogeneous, even in the case of short exposure times. Indeed, the atmospheric turbulence (tip-tilt component) is frozen, but local defocusing and distortions still exist. Today, they could be corrected by adaptive optics. The field of view is about 75 x 100 arc seconds (after Janssen's "Atlas de photographies solaires", 1903).

THE SOLAR SPECTRUM AND THE MONT BLANC ADVENTURE (1888-1895)

Janssen, besides three decades of solar imagery, studied in parallel the solar spectrum. In 1862, he observed details of the Brewster bands of di-oxygen O2, in the red and near infrared part of the spectrum, namely the A band (760 nm wavelength) and the B band (680 nm). An important question arose: is there a contribution of the Sun to the O2 spectral lines, or are they strictly of telluric origin ? We know today that oxygen is present in the solar atmosphere, but not in the di-atomic form (O2). It appears in some molecules such as CO, SO, OH, metallic oxides such as SiO or TiO, and is among the most abundant elements in the solar atmosphere, together with carbon, nitrogen, neon and many metals. The abundance is of course far from hydrogen and helium (the main elements). Oxygen also exists in the mono-atomic form (O) and is revealed by the ultraviolet spectrum of the million degrees solar corona (the external envelope), as neutral (OI) or in various states of ionization, from OII (1 lost electron) to OVIII (7 lost electrons), depending on temperature conditions. Of course, these properties were unknown in Janssen's epoch. He first found evidence of the B band of O2 in the Meudon spectroscopic laboratory. In 1889, the Eiffel tower in Paris was completed. Janssen observed from Meudon the light beam of this giant light-house, at a distance of 7700 m, equivalent to the thickness of the Earth's atmosphere, and saw the absorption lines of atmospheric O2. An experiment, made at Meudon with a 60 m pipe filled with pure di-oxygen at the pressure of 28 bars, equivalent to the atmospheric thickness, showed clearly the A and B bands. Even with a 120 m pipe of pure O2 at the pressure of 1 bar and equivalent to only 1/10 of the atmospheric thickness, the B band appeared. However, if there was no doubt that O2 lines of the solar spectrum mainly originate in the air-mass above the observer, a contribution of the Sun might exist and stay hidden. The only way to resolve this question consisted of observing the solar spectrum at the highest possible altitude: this was the starting point of the Mont Blanc adventure [START_REF] Malherbe | Les observatoires du Mont Blanc en 1900[END_REF][START_REF] Grévoz | Le pari fou de Jules Janssen, le savant épris du Mont Blanc[END_REF].

Figure 10: The north face of Mont Blanc and the route to the summit across the glaciers. R1 is the "Grands Mulets" refuge (3050 m) where first spectroscopic observations were done by Janssen in 1888. R2 is the "Grand Rocher Rouge" hut (4500 m) especially built in 1893 as a relay for the construction of the observatory (the "Corridor" is the historical route of the first ascent of 1786, but never used today). R3 is the refuge and geophysical observatory of J. Vallot (4360 m), erected on the "Rochers foudroyés". Janssen visited R1 and R3 in 1890, and R1 and R2 during his ascents of 1893 and 1895 (courtesy Paris Observatory).

In 1887, Janssen went to the Pic du Midi de Bigorre (2870 m), where an observatory was founded in 1875 by Charles Champion de Nansouty (1815-1895) and Xavier Vaussenat (1831-1891) for meteorological purposes (astronomy came later under the auspices of Benjamin Baillaud, 1848Baillaud, -1934)). Janssen always travelled with a standard spectroscope manufactured by Jules Duboscq (1817-1886), in order to facilitate the comparison of results obtained in different places and elevations. He sometimes used, in addition, more dispersive instruments. One year after observations at Pic du Midi, he decided to approach Mont Blanc and first went to the "Grands Mulets" refuge (R1 in Figure 10).

4.1

The first approach of high altitude: the ascent of the "Grands Mulets" (1888) For Janssen, going to the "Grands Mulets" (3050 m), the main step along the Mont Blanc route from Chamonix, was much more complicated than going to Pic du Midi (a region of medium mountains accessible in summer by trails), because the refuge is located among chaotic glaciers, particularly rugged in autumn. Large and deep crevasses are open, and finding a path, even for mountain guides, is a complicated task [START_REF] Janssen | Sur le spectre tellurique dans les hautes stations et en particulier sur le spectre de l'oxygène[END_REF]. Moreover, Janssen was handicapped physically by his claudication since childhood (consecutive to an injury) and had to be carried during the ascent. It took two days from Chamonix, instead of one day for experienced alpinists. Janssen chose October in order to benefit from very cold and dry air conditions, which reduce the intensity of atmospheric lines (water vapor) in the solar spectrum. The first day, Janssen travelled on the back of a mule, which followed the trail from Chamonix to the "Pierre pointue" chalet (2300 m). The second day was much more difficult. There is never a permanent trail across glaciers and the guides have to find a complicated path among huge crevasses. Janssen imagined an appropriate means of transport: the ladder-chair (Figure 11). The astronomer was carried by four or six guides and the ascent was so long that it was not until night that the caravan arrived at the refuge (Figure 12). Janssen used the Duboscq spectroscope, which was composed of three arms and two prisms. The first arm contained the adjustable slit and the collimator. The second was a chamber, with a focusing lens and an eyepiece to observe the spectrum. The third arm projected a measurement scale onto the prisms. The spectrograph was fed by a Prazmowski heliostat (Figure 12). Janssen observed at the "Grands Mulets" a marked decrease in intensity of O2 lines (the B group), but it was not sufficient to conclude that O2 does not exist in the Sun's atmosphere. It was necessary to reach higher altitudes and install a new station with permanent and powerful scientific equipment, in good observing conditions: this was the birth of the observatory project.

Figure 11: In order to cross the crevassed glaciers, Janssen designed a 4 m ladder-chair with four carriers for his ascents to the "Grands Mulets" refuge (3050 m). The crevasses are so numerous and so large along this route that it is impossible to use a sleigh (drawing after a photography of E. Whymper, Janssen, 1895).

The first ascent of Mont Blanc (1890)

Jospeh Vallot (1854Vallot ( -1925) ) erected in 1890 a laboratory on Mont Blanc slopes [START_REF] Vallot | Construction de l'Observatoire du Mont Blanc[END_REF], at 4360 m elevation (R3 in Figure 10). It was a multi-purpose station to study meteorology, geophysics, medicine, cartography and glaciology, but astronomy was not concerned. In order to observe the solar spectrum at the summit of the mountain, Janssen decided to follow the R1-R3 route of Figure 10. took several days to reach the summit (Janssen, 1890a). Mules were used between Chamonix and the glaciers. Then, Janssen was carried up to R1 (the "Grands Mulets") with the ladder-chair, as in 1888. Above R1, the glaciers are smoother and Janssen imagined another means of transport, a special sleigh adapted to the constraints of high altitude (Figures 13 and14). Many guides were necessary to pull up the astronomer from R1 to the Vallot laboratory at R3. Janssen stayed there three days with Vallot, because of bad weather. The summit of Mont Blanc was reached successfully with the sleigh and twelve guides ("my twelve apostles", as Janssen said). The ascent was particularly difficult along the narrow ice ridge leading to the summit. Janssen observed rapidly the solar spectrum with the Duboscq spectroscope, still revealing the B group of di-oxygen, but with strongly reduced intensity.

Figure 13: Left: the mountain guides of Chamonix. The special sleigh was used above the "Grands Mulets" refuge where the glacier is easier, with few crevasses. Janssen was accompanied during this expedition by 22 persons, twelve of whom were guides. Right: Janssen's sleigh above the refuge on the way to the Mont Blanc. The guides used a rope ladder to pull up the sleigh. For the ascents of 1893 and 1895, they sometimes used winches, especially designed by Janssen, which were anchored in the snow or ice by picks (drawings after photographs, Janssen, 1890b).

The observatory and second ascent of Mont Blanc (1893)

The laboratory of J. Vallot, erected in 1890 on rocks located 450 m below the summit (the "Rochers foudroyés", R3), was a nice structure for the study of earth sciences, such as glaciology, but astronomy was outside its scope. For celestial observations, a clear horizon is required, and the flanks of mountains are not convenient. Moreover, they generate atmospheric turbulence. At the summit, one can expect laminar air flows, in favour of good images and seeing. For these reasons, astronomers prefer the top of high mountains. Janssen naturally targeted the Mont Blanc summit. This logical choice, not well understood in the valley, was interpreted as a competition between both characters.

The success of the previous ascent (1890) was reported to the French Academy of Sciences, which encouraged the project of a high altitude astronomical observatory. Several sponsors, friends of the sciences, such as Raphaël Bischoffsheim (1823-1906), Prince Roland Bonaparte (1858-1924), Baron Alphonse de Rothschild (1827Rothschild ( -1905) ) brought more than 250 000 francs and a society was founded with former minister of finance Léon Say (1826-1896) acting as the president. He was charged with negotiating an annual grant from the government.

However, the summit of Mont Blanc is not an hospitable place ! It is covered by ice and snow and is subject to terrible storms and winds. There is no rock to establish the foundation of a sustainable construction. The famous engineer Gustave Eiffel (1832Eiffel ( -1923)), one of Janssen's friends, promised to erect a metallic laboratory, but only if rocks could be found 12 m below the iced summit. The Swiss engineer Xavier Imfeld (1853-1909), a topographer, was charged in 1891 with ice soundings. Two horizontal tunnels of 23 m length were excavated to search for rocks, without any success [START_REF] Durier | Les travaux de M. Eiffel au Mont Blanc[END_REF]. Eiffel withdrew from the project, and Janssen reconsidered it on a new basis.

The idea was now to put the observatory directly on ice. But it was necessary to test whether the resistance of the snow was sufficient to support the weight of the construction. An experiment was undertaken at Meudon in 1892 during the winter. A pile of snow was packed in order to reach the density measured at the summit of Mont Blanc. Then, a column of 360 kg of lead was elevated above, corresponding to a pressure of 4000 kg/m². The settling was evaluated at 8 mm only, so that a construction of 5 m x 10 m size, weighing a few tens of tons, made of wood, appeared quite reasonable. The shape was a truncated pyramid [START_REF] Tissandier | L'Observatoire du Mont Blanc[END_REF], with the lower part buried into the snow in order to resist the violent storms which occur at high altitude. In order to deal with horizontality fluctuations, in particular under the pressure and motion of ice, several actuators were incorporated at the base of the wooden structure (Figure 15). The observatory was built in Meudon under the supervision of the architect Emile Vaudremer . It was transported to Chamonix by train in several parts and divided into 800 man loads. The route to Mont Blanc was divided into four sections and a new hut (R2 in Figure 10) was built during the 1892 summer at the "Grand Rocher Rouge". The first section was Chamonix-glacier (with the help of mules), and the three other sections (glacier-R1 refuge, R1-R2 hut, R2summit) were only man-portable. The R2 hut (4500 m) was strategic, because it was used by the carpenters during the erection of the observatory, both for lodging and storage of materials and tools. The 1892 summer was used for carriage and the beginning of the construction, which was finished in 1893. Winches were useful to raise the heaviest charges, and Janssen, in September 1893, planed his second ascent to inaugurate the observatory (Figures 16 and17). He chose the itinerary R1-R2 (Figure 10) followed during the construction, and the winches, still in place, helped the guides to lift the traveller's sleigh [START_REF] Janssen | L'observatoire du Mont Blanc[END_REF].

Janssen brought the Duboscq spectroscope and a second spectrograph with better dispersion, able to resolve the doublets of the B group of O2. It was composed of a 5 cm x 8 cm, 568 groves/mm Rowland and longer focal length lenses. The focal length of the collimator and of the photographic chamber were respectively 1.10 m and 1.50 m, and their diameter was 10.8 cm. A 0.85 m focal length objective formed a 7.6 mm diameter solar image on the 20 micrometre spectrograph slit. A spectrum of the B band of O2 obtained with this spectrograph is shown in Figure 18.

In 1894, a long-running meteorograph (Figure 19) was manufactured by Jules Richard (1848-1930) and mounted at the observatory in 1895 [START_REF] Janssen | Météorographe à longue marche de l'Observatoire du Mont Blanc[END_REF]. It had, in principle, 8 months of autonomy in order to pass the winter, a period where ascents are impossible. It was driven by a clock mechanism with a 90 kg counterweight descending several meters. This high precision instrument was able to record, on paper rolls, temperature, pressure, hygrometry, wind speed and direction. However, the climatic conditions were extremely severe for the meteorograph, so that this high performance, but very delicate machine, never worked for very long.

Figure 15: Mont Blanc Observatory (5 m x 10 m base) was designed, mounted and tested in Meudon, then dismounted and transported to Chamonix by train. The 15 tons of wood were conveyed to the summit of Mont Blanc by many carriers. Eight hundred loads were necessary, and the heaviest parts of the framework were hauled with winches. At the summit of Mont Blanc, the first floor was buried into the snow in order to resist the winds and storms, which can be extremely violent at this altitude. The structure was supported by several actuators in order to keep it horizontal and compensate for ice movements. A central spiral staircase (not represented) provided access to the floors and the meteorology tower. The Observatory was covered by double walls (After Tissandier, 1893, left, and courtesy Paris Observatory, right). The main astronomical equipment was a large refractor designed in Meudon, transported to the summit in 1895 and assembled there in 1896. This ambitious instrument [START_REF] Millochau | La grande lunette de l'observatoire du Mont Blanc[END_REF] was composed of a static external tube of 70 cm diameter enclosing the rotating internal tube (35 cm diameter) of the refractor, supporting a 30 cm objective (two lenses of 5.40 m focal length) made by the Henry brothers of Paris Observatory. The axis of the metallic tubes was parallel to the Earth's polar axis. The internal tube was mounted on rollers and there was a clock-type mechanical motor. The diameter of the solar image at the focus was 49.4 mm. The refractor was fed by a polar siderostat, with a flat mirror of 60 cm diameter. This mirror was removable, so that when the refractor was not used, it was protected by a metallic cover. The refractor was built and tested in Meudon (Figure 20). It was then dismounted, segmented into several parts and transported to the summit of Mont Blanc under the form of many carrier charges, not exceeding 30 kg. The heaviest charge was the mirror of the siderostat (62 kg). At the focus, micrometric eyepieces and photographic plates were available. It was possible to catch a celestial object using coordinates, with two graduated circles. The first one was attached to the refractor (right ascension), while the second one was fixed to the flat mirror (declination). There was a cranked mechanical transmission to adjust the inclination of the flat mirror from the observing room. A small telescope (8 cm diameter) was attached to the 30 cm main refractor to facilitate the pointing of celestial objects, and a second auxiliary telescope, with larger field of view, was also present.

A small removable spectrograph was mounted later, in 1904 (Figure 20), with the following characteristics: 60 cm focal length collimator and two chamber objectives of either 30 cm or 60 cm focal length. The dispersive element was a flint prism. The height of the slit was 9 mm. The polished nickel of the slit reflected light towards a small control telescope focused on the slit jaw. Photographic plates were available. The spectrograph permitted, for instance, the measurement of the width of the solar chromosphere (2900 km) which is not visible in white light, but only in some Fraunhofer lines such as Hα or CaII H and K.

The observer stood inside the observatory, on the lower level. As the refractor was long (6 m), the northern wall of the upper level was open (Figures 21 and22) and the 30 cm objective was permanently outside. A platform was constructed to access the cover of the tube and install or remove the polar siderostat. Such a powerful and outstanding astronomical instrument was really unexpected at 4810 m, among glaciers, ice and cold ! Apart the major difficulties of ascents (the summit is 3800 m above Chamonix), the most important obstacles were meteorological conditions: strong winds, thunderstorms and atmospheric electricity, powdery snow and frost, and condensation of water vapor on optical pieces forming frost. But no rust was observed, because it never rains at this altitude and the air remains always dry.

Janssen saw the refractor at the summit in disassembled parts during his last ascent (Janssen, 1895a and1895b). For a 71 year-old person, constrained to be carried by guides or pulled up with a sleigh, it was an exploit. Janssen did not himself use this incredible instrument, because it only became operational one year later, in 1896. He examined the lines of the solar spectrum with a two-prism arrangement of the Duboscq spectroscope fed by a heliostat. He concluded that "the presence of oxygen and water in the solar envelope, and their respective physical state, seem to be answered questions, but this problem is so crucial that observations need to be repeated". Janssen also noticed an almost imperceptible displacement of the observatory, which was expected.

Activities of the observatory after 1895

About 25 researchers visited the observatory (Figures 23 and24), either for solar or night observations with the 30 cm refractor, and an associated spectrograph installed in 1904 by Gaston Millochau (1866-1922), a Meudon astronomer. During ten years, between 1896 and 1906, about five scientific expeditions per year were undertaken. A famous visitor, Milan Stefanik (1880Stefanik ( -1919)), one of the co-founders of the Czechoslovakian Republic, was an astronomer and collaborated with Janssen at Meudon. He organized during seven years observations at Mont Blanc, and beat the record of length for a stay at the summit, together with Millochau, with 13 consecutive days. Aleksey Hansky (1870Hansky ( -1908)), who became an astronomer at the Poulkovo Observatory, also worked with Janssen, and made many successful ascents over eight years. He stayed up to 12 consecutive days at the summit.

Most research and results were summarized by [START_REF] Radau | L'astronomie au Mont Blanc[END_REF]. Many astronomical observations were undertaken, besides the studies of the visible solar spectrum by Janssen, which were continued in 1898 by Aymar de La Baume-Pluvinel (1860-1938). We can cite the investigations of the zodiacal light, the imaging of Venus and Mercury, the study of the near infrared and ultraviolet solar spectrum and some attempts at observations of the hot solar corona outside eclipses (which will not be successful before the Lyot's coronagraph, see Dollfus, 1983). Measurements of the total solar irradiance (TSI) were performed with the actinometers of André Crova . The TSI was at that epoch called the "solar constant" (this is the solar flux entering the atmosphere, about 1360 W/m², we know today that it fluctuates during the eleven-year solar cycle). Hansky tried to improve the precision of the pyrrheliometer of Claude Pouillet (1790-1868), with the new pyrometer telescope of Charles Féry (1865Féry ( -1935)). He conducted several observations between 1898 and 1904 which led to a more precise value for the effective temperature of the Sun.

In meteorology, although this was far from the main goal, some observations concerning atmospheric electricity and ozone can be mentioned. The observatory, without any satisfying ground conductor, was not protected against lightning strikes, in contrast to the Vallot laboratory which was almost a Faraday cage, so that atmospheric phenomena were impressive. The thermometer recorded temperatures as low as -45°C in winter 1901, but the long-running meteorograph never worked properly in these difficult conditions. Miscellaneous research in physics, such as gravimetry with astronomer Guillaume Bigourdan (1851Bigourdan ( -1932)), electrical conductivity of the snow, telegraphy with wires put on the glaciers between the "Grands Mulets" refuge and Mont Blanc, medicine and biology, were also undertaken at the observatory, and were reported by Janssen in the second volume of "Annales de l'Observatoire d'astronomie physique de Paris, sis parc de Meudon" in 1906.

The decline of the observatory (1907-1909)

After his last ascent (1895), Janssen went to Chamonix in 1897 to organize an expedition, and later collaborated from Meudon in scientific missions until his death in 1907. However, the observatory began to sink slowly into the snow (Figure 25). The manoeuvre of actuators (1904,1906) was efficient until the formation of a crevasse below the observatory; it soon became displaced, uninhabitable and dangerous. In 1909 (Figure

Figure 2 :

 2 Figure 2: The ruins of Meudon chateau in 1871 (left) and the restored and transformed chateau in 1893 (right) with the new 18.5 m dome. The red boxes (left) indicate the parts which were completely destroyed by fire and suppressed (after Janssen, 1896).

Figure 4 :

 4 Figure 4: The dome of the 100 cm diameter Newtonian telescope at F/3 (3 m focal length), in 1893. It was changed into a Cassegrain telescope in 1970 to increase the focal length to 22 m (after Janssen, 1896).

Figure 5 :

 5 Figure 5: The temporary installation of solar refractors at Meudon Observatory in 1878. At left, the Adam Prazmowski refractor, at right the Carl Steinhel refractor used for The Venus transit of 1874 and a small temporary 5 m diameter dome (courtesy Paris Observatory).

Figure 8 :

 8 Figure 8: A large sunspot group observed by Janssen on 22 June 1885 with photographic emulsions sensitive to the blue part of the solar spectrum. Sunspots have a size comparable to the Earth, are related to solar activity and cyclicity, and exhibit strong magnetic fields (0.2 tesla). The granulation is also well visible in the upper part. The field of view is about 150 x 180 arc seconds (courtesy Paris Observatory).

Figure 9 :

 9 Figure 9: The full Sun observed by Janssen on 2 June 1893 showing sunspot groups and the shutter located at the primary focus of the refractor (at right), allowing extremely short exposure times to freeze the turbulence (courtesy Paris Observatory).

Figure 12 :

 12 Figure 12: Left: the refuge "Les Grands Mulets", 3050 m, the main station on the route to Mont Blanc at Janssen's time. It is located on a rocky spur surrounded by steep glaciers with large and deep crevasses. The refuge was visited four times by Janssen, in 1888, for spectroscopic observations, and in 1890, 1893 and 1895 for his three ascents of the summit (author's collection). Right: the Prazmowski heliostat to catch the solar light and a portable Duboscq spectroscope (courtesy CNUM/CNAM).

Figure 14 :

 14 Figure 14: Janssen's sleigh (top of the photography) waiting to cross a large crevasse on a ladder. Deep crevasses similar to this one are numerous and dangerous on the route to the Mont Blanc (author's collection).

Figure 16 :

 16 Figure 16: Janssen's Observatory in 1893 at the summit of Mont Blanc (author's collection).

Figure 17 :

 17 Figure 17: Janssen's Observatory at the summit of Mont Blanc, a few years after the inauguration of 1893 (courtesy Paris Observatory).

Figure 18 :

 18 Figure 18: Lower panel: the solar spectrum obtained at Mont Blanc with the Rowland grating spectrograph. The B band of oxygen spectral lines (O2 molecule) appears at the wavelength of 680 nm, in the red. Upper panel: the spectrum of the Jungfraujoch (3600 m) for comparison, degraded to almost the same resolution and observed by Delbouille et al (1973) with wavelength indicated in Angström (courtesy Paris Observatory).

Figure 19 :

 19 Figure 19: The long-running meteorograph manufactured by J. Richard for the Mont Blanc observatory in 1894. A = clock able to work during 8 months. B = barometer and graph. C = mercury column. D = anemometer and graph. E = thermometer and graph. F = hygrometer and graph. G = the counterweights of the paper rolls. H = clock pendulum. I = clock transmission axis to the instruments (drawing, after Janssen, 1894).

4. 4

 4 The 30 cm refractor and the third ascent of Mont Blanc (1895)

Figure 20 :

 20 Figure 20: The 30 cm diameter, 5.40 m focal length refractor (left) mounted and tested at Meudon in 1895. The two-lens objective was made by the Henry brothers of Paris observatory. The direction of the tube was parallel to the earth rotation axis. The refractor was fed by a 60 cm polar siderostat (flat mirror on top, left).This mirror was manually removed and replaced by a cover outside observing sessions. The refractor was installed at Mont Blanc during the summers of 1895 and 1896. A 0.60 m focal length spectrograph (centre) was available at the focus of the telescope in 1904. At right, the refractor in imagery mode with the eyepiece holder (after[START_REF] Millochau | La grande lunette de l'observatoire du Mont Blanc[END_REF][START_REF] De Fonvielle | A l'Observatoire du Mont Blanc[END_REF].

Figure 21 :

 21 Figure 21: The 30 cm fixed-direction refractor at Mont Blanc Observatory was operational in 1896. An opening was created for the tube in the northern wall. The telescope just rotated around its axis (right ascension) to compensate for diurnal motion. The platform was used to install and remove the flat mirror of the 60 cm polar siderostat to catch the light and adjust the declination (courtesy Paris Observatory).

Figure 22 :

 22 Figure 22: Janssen's Observatory at the summit of Mont Blanc, with the meteorological turret. The 30 cm diameter, 5.40 m focal length refractor was only available in 1896 (compare to Figures 16 and 23). The platform giving access to the cover of the refractor is located north of the construction and is visible at left. The refractor is parallel to the Earth's rotation axis and fed by a 60 cm polar siderostat. It was necessary to install the flat mirror before observations (courtesy Paris Observatory).

Figure 23 :

 23 Figure23: The Mont Blanc Observatory, west side. In both pictures, the 60 cm polar siderostat is mounted in front of the 30 cm objective, at left. The cover of the refractor lies on the platform (courtesy Paris Observatory).

Figure 24 :

 24 Figure 24: Observations, measurements, experiments and life at the Mont Blanc Observatory (courtesy Paris Observatory and La Nature).

CONCLUSION: JANSSEN'S HERITAGE

During two centuries, from 1667, Paris Observatory was mainly devoted to celestial mechanics and astrometry. Janssen introduced physical astronomy in France, following similar movements in Europe and in the USA. Meudon Observatory was initially named "Observatoire d'astronomie physique", which indicates clearly the aim of the founder in 1875. Janssen combined two major techniques, photography and spectroscopy, applied to the study of the physical and chemical nature of celestial objects, in particular the Sun. Meudon and Paris observatories amalgamated in 1926 and belong today to the oldest and biggest astronomical institutions in the world. The Mont Blanc adventure was motivated by the question of the nature (solar or terrestrial) of di-oxygen lines in the solar spectrum, at an epoch when stratospheric balloons, rockets or satellites did not exist (although some balloons already reached higher elevations than Mont Blanc). Indeed, 50% of the atmosphere is below the summit of the mountain. Janssen could not look for durability when he decided to put the observatory directly on ice, in contrast to average altitude observatories created at the same moment. This audacious project consisted also of exploring the potential of high altitude for other astronomical questions. In that sense, Janssen can be considered as a precursor of modern astronomy, involving, on one hand large urban and multi-purpose institutes, and on the other hand many dedicated instruments in much better sites or space missions which eliminate atmospheric absorption, and rarely exceed 20 years of lifetime. Hence, the heritage of Janssen is not only the development of physical astronomy and associated techniques, but also a new vision of astronomical projects based on specific and short duration instruments. Janssen was a specialist of the Sun: space solar missions, such as SOHO and Solar Orbiter, are the continuation of this strategy at the European scale. The Mont Blanc summit was not suitable for permanent stations, but recent and similar high altitude observatories located in drier sites, such as Hawaii (4200 m, optical-telescopes) or Atacama (5100 m, radio-telescopes), with easier access, form a continuity. Janssen concluded in 1900: "Progress in science needs to specialize instruments, increase their size, and locate them in places where atmospheric troubles are minimized, leading inevitably to well chosen and high altitude stations".

ACKNOWLEGMENTS

The author wishes to kindly thank the three referees, among them Dr James Lequeux and Dr William Tobin, and the Editor, Pr Wayne Orchiston, for extremely helpful corrections, comments and suggestions.

ON-LINE SUPPLEMENT MATERIAL (MPEG 4 VIDEO MOVIE, JPEG IMAGES)

This short MPEG 4 video movie (2 min 30 s) shows a scientific ascent of Mont Blanc in 1900 at the epoch of Janssen's Observatory. This expedition follows the R1-R3 route (Figure 10) across glaciers. The movie starts at the entry of the glaciers (2400 m) and ends at Janssen's Observatory (after an old 9.5 mm black and white movie, author's collection). It is available here (low quality only): https://drive.google.com/file/d/1mnpz_fFKQuYwaDcmeSNr1nSwa0rp-y2r/view?usp=sharing The figures of this paper are available in JPEG here: https://drive.google.com/drive/folders/1OCg2sz7J9THyz-ShyS1AaFLSKD5kPpAo?usp=sharing A remark: today, the French normal way of ascent starts à the "Nid d'aigle", a station (2370 m) at the end of a rack-railway coming from Saint Gervais. This is the "Aiguille du Goûter" route, open later than the historical route from Chamonix (used by Janssen), but faster and avoiding the falling and dangerous seracs of the "Dôme du Goûter". The Italian normal way was open by the pope Achille Ratti in 1890 (the "Aiguilles Grises" route) and merges with the French one at "Col du Dôme" (4250 m), just below Vallot's Observatory which still exists.

Most papers published at the end of the XIX th century or at the beginning of the XX th century were written in French language. Janssen's monumental work was compiled by H. Dehérain in two volumes (1929, 1930). Dehérain, H., 1929Dehérain, H., , 1930. . Oeuvres scientifiques de Jules Janssen (two volumes). Société d'éditions géographiques, maritimes et coloniales, Paris. On-line at CNUM/CNAM. http://cnum.cnam.fr/redir?8RA35.1 (volume 1, 1929) and http://cnum.cnam.fr/redir?8RA35.2 (volume 2, 1930).