1876.] 



President's Address. 



359 



advantage (I allude to the " Direct-vision Spectroscope," long asso- 

 ciated with the name of Hofmann, because that optician was employed 

 by Jansseo), and pointed out how spectrum-anah'sis might enable us to 

 settle the vexed question of the existence of a lunar atmosphere. 



In 18G6, with reference to the telluric hue, he experimented at La 

 Yillette on a tube, some 37 metres long, containing steam at the pressure 

 of 7 atmospheres, with the result that by comparing the spectra he was 

 enabled to demonstrate that the telluric lines were really due to the 

 absorption of aqueous vapour. The experiment was repeated in another 

 form by observations of the spectrum of a flame se"\xral miles away 

 through the vapour overling the Lake of Geneva. 



After these researches he sought and obtained a mission from the 

 Paris Academy of Sciences to South-eastern Europe to make observations 

 on the spectra of stars ; and he was enabled to establish the fact that 

 aqueous vapour exists in the atmosphere of some of them. 



These researches on aqueous vapour led him to observe many spectra, 

 among which were iodine, bromine, and others ; and in the ' Comptes 

 Bendus ' and Proceedings of the Societe Philomathique, observations are 



o 



recorded which show that at that time he and Angstrom were in the van 

 of such researches. 



Jaussen's observations regarding aqueous vapour naturally led him to 

 take every occasion of studying the solar atmosphere ; and since the 

 annular echpse of 1867 (which he observed at Trani) there has been one 

 total solar eclipse only which he has not studied. 



After a scientific mission to the Azores in 1867, he went to India in 1868 

 to observe the great eclipse of that year. Xot only were his observations 

 of the eclipse itself of the highest value, but during the eclipse, with 

 a flash of genius, the thought occurred to him that an eclipse was not 

 necessary to the observation of the social phenomena into which every- 

 body was inquii'ing ; and he was the first to apply the method, now well 

 known, which is being utilized in all civihzed countries for the advance- 

 ment of knowledge. 



For some months after the eclipse Janssen remained in India, and 

 brought home a rich series of obserA'ations, opening up many branches 

 of inquiry which have since proved most fruitful in result. 



In the echpses of 1870 in Africa, 1871 in India, and 1875 in Siam, 

 Janssen was present, and advanced further the question A^hich he had 

 set himself, and with the solution of which his name will always be 

 associated. 



Janssen's skill as an observer and his sound knowledge of optical and 

 mechanical questions, have not been shown merely in connexion with the 

 spectroscope ; he was anxious to observe not only the recent transit of 

 Venus, but to obtain records of several physical phenomena which can 

 be observed only at such times. Por this purpose he gave attention to 

 astronomical photography ; and the result was the introduction of his 



