January 9, 1908] 



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HR. p. J. C. JANSSEN. 

 ■p R.\NCE is again called upon to mourn the loss. 

 ^ of a veteran astronomer whose services have 

 rendered him conspicuous among the many eminent 

 men of science his country has given to the world. 

 Jules Janssen, born in 1824, was first a painter, but 

 for more than fifty years contributed to the scientific 

 literature of his country and enriched many depart- 

 ments of physics by his untiring energy, his accurate 

 observations, and his fertility of resource. He was a 

 great traveller ; his first scientific expedition was in 

 1S57 to Peru, to study the magnetic equator. Ten years 

 after he was studying the eruption of Santorin with 

 Kouque. It is not possible to do justice to his work 

 within a small compass, but fortunately its salient 

 features will long remain fresh in our memories. 



janssen 's reputation will rest mainly upon his 

 numerous and important researches on light spectra, 

 and the methods he employed for pursuing his investi- 

 gations. He, early recognised the power of the spec- 

 troscope as an engine for research, and in its applica- 

 tion to many problems connected with solar activity 

 he was without rival among his own countrymen. In 

 1862, he published the first results of his celebrated 

 researches on the origin of the telluric lines in the 

 solar spectrum, and it is only necessary to recall the 

 history of spectroscopy in the last half-century to 

 make us appreciate the value of his pioneering ser- 

 vices in this direction. The perseverance and ardour 

 with which he pursued this subject was shown, first 

 at La Villette, where he so arranged his experiments 

 that the light the spectrum of which was to be exam- 

 ined passed through a tube, 37 metres long, containing 

 steam under a pressure of seven atmospheres, and 

 later, at Geneva, where in another series of experi- 

 ments the light traversed several miles of atmosphere 

 immediately overlying the lake. For many years 

 the same problem in different aspects occupied his 

 attention. 



Long after the part played by water vapour in 

 modifying the spectrum had been settled, Dr. Janssen 

 had recourse to experiments of the same nature in 

 order to decide the precise character of the spectrum 

 effects due to oxygen. The pursuit of this question 

 and his anxiety to vary as much as possible the con- 

 ditions under which his observations were made, 

 carried him to the top of Mont Blanc, where his ex- 

 periments warranted him in asserting that there was 

 no evidence of the presence of oxygen in the exterior 

 and cooler parts of the solar atmosphere. To speak 

 with equal confidence of the lower, and, consequently, 

 hotter layers, it was necessary to examine the spec- 

 trum of oxygen when submitted to high temperatures 

 and great pressure. By ingenious devices. Dr. Janssen 

 succeeded in raising oxygen to a temperature of Soo° 

 or 900°, and in placing the gas under a pressure of 

 1000 atmospheres. As before, his observations pointed 

 to the absence of oxygen in the sun's atmosphere. 

 His researches on the effect of planetary atmospheres 

 in modifying the spectrum of solar light were equally 

 thorough and satisfactory. 



The observations made on Mont Blanc firmly con- 

 vinced Dr. Janssen of the advantages offered by high 

 mountains for the conduct of certain inquiries, in 



NO. 1993, VOL. yj] 



which it was of importance to reduce as far as pos- 

 sible the thickness of the atmosphere, through which 

 observations had to be made. In spite of increasing 

 age and the inconvenience of lameness, he interested 

 himself energetically in the construction of the Mont 

 Blanc Observatory, and on more than one occasion 

 personally made the ascent of the mountain to assure 

 himself of the success of the enterprise. Under his 

 auspices various physical inquiries have been success- 

 fully pursued in this elevated observatory. 



As an observer of the sun at the time of total solar 

 eclipse. Dr. Janssen was indefatigable. In 1868, he 

 began that long series of observations which have so 

 much enriched our knowledge of the sun's surround- 

 ings. For it was while observing this eclipse at 

 Guntoor that he was impressed with the possibility of 

 observing the prominence lines on the limb of an un- 

 eclipsed sun. How, without delay, he put his plan 

 into operation and enjoyed the advantage of a pro- 

 longed eclipse is well known, as also the fact that the 

 announcement of his discovery reached the Paris 

 Academy of Sciences at the same time as a similar 

 contribution from Mr. (afterwards Sir Norman) 

 Lockyer informed that body of the successful results 

 of his more prolonged researches. The claims of 

 each have been fully admitted, and just as the names 

 of Adams and Le Verrier are connected with a famous 

 problem in gravitational astronomy, so those of 

 Lockyer and Janssen are joined in the solution of a 

 fundamental problem of physical astronomy. A medal 

 containing effigies of the heads of the two astronomers 

 side by side was struck by the French Government to 

 commemorate this " Janssen-Lockyer Discovery." 

 This is an oft-tale tale, and it would not be necessary 

 to repeat it here but for one circumstance which is not 

 so well known, and which it is desirable to emphasise. 

 This is the generous recognition which Dr. Janssen 

 ever expressed towards his English confrere, and his 

 ready acknowledgment of the value of English work. 

 Fortunately, the days of international jealousy in 

 science have passed, but the loyal and hearty appre- 

 ciation which Dr. Janssen exhibited stands out as a 

 conspicuous example of unselfish and kindly interest, 

 in which no unworthy considerations found a place. 

 Needless to say that his warm-hearted sympathy and 

 encouragement was highly valued and cordially 

 reciprocated. 



At Sir Norman Lockyer 's suggestion, Janssen was 

 invited to join the English Eclipse Expedition of 

 1870, and as he was then in besieged Paris, thanks 

 to the exertions of the English Foreign Office the 

 invitation reached him there by the hands of Bis- 

 marck, who accompanied it with a safe conduct. This 

 he declined, and left the beleaguered city in a bal- 

 loon. On that occasion he carried with him 

 the essential parts of a reflector especially con- 

 structed to collect evidence about the solar corona. 

 He repaired to Oran, and deserved better fortune 

 than to find the sky completely obscured by clouds at 

 the time of the eclipse. In 1871 and 1875 he was 

 again in .-\sia, taking part in the observation of solar 

 eclipses, while in 1883 he was one of that remarkable 

 partv of enthusiasts who repaired to the lonely coral 

 reef in the Pacific known as Caroline Island. For 

 this eclipse Dr. Janssen used telescopes of six and 

 eight inches aperture, and on his photographs ob- 

 tained an extension of the corona further than it could 

 be traced in the field of the telescope, revealing a 

 remarkable complexity of structure. Here, too, he 

 confirmed his previous suspicions of the presence of 

 reflected Fraunhofer lines in the spectrum of the 

 corona. His passionate interest in solar phenomena 

 never deserted him, and on the occasion of the eclipse 

 in 1905, notwithstanding his advanced age, he was 



