1 86 Obituary Notifies of Fellows deceased. 



They are to be found in Vol. I. of the Journal de Physique, then recently 

 founded by his friend d' Almeida. 



For several years subsequently, and' at different times, Cornu was 

 occupied with researches on the spectrum. He measured the wave- 

 lengths of the hydrogen rays with a precision previously unknown, 

 enabling a comparison to be made between the values so obtained by 

 experiment and the theoretical formulae which had been proposed by 

 Balmer and others to express them. The suggestions of Dr. Johnstone 

 Stoney, and the later developments of Kayser and Kunge, will not be 

 forgotten in this relation. His special and searching inquiry into the 

 ultra-violet solar spectrum is also memorable. He made observations 

 on atmospheric absorption in the spectrum, using photographic- 

 methods, at his country house at Courtenay, where he spent most of 

 his vacations. He devised an elegant modification of the slit 

 apparatus to enable simultaneous observations to be made of light 

 from the two ends of a diameter of the sun's surface, the advancing 

 and the retreating, and thus, by an application of Doppler's principle, 

 was able effectually to pick out those lines which were of solar origin 

 from those due to absorption in the stationary atmosphere of the 

 earth. He was able to fix the inferior limit to the ultra-violet end of 

 the spectrum, so far as it is visible at low elevations, and found that 

 in the laboratory air is opaque to ultra-violet waves of a lesser wave- 

 length than 0'185/A. His work on meteorological optics has thus been 

 summarised by M. Guillaume : " Such researches, in the course of 

 which he was often led to a scrutiny of the sky, could not fail to draw 

 his attention to the optical phenomena of the atmosphere, the study of 

 which, though energetically pursued by the French physicists of last 

 century, is to-day somewhat neglected. The splendid glows which 

 were observed in the sky toward the end of 1883 furnished to Cornu 

 an occasion to utilise the profound knowledge which he possessed of 

 the phenomena of optics. He showed that the twilight glow, which 

 at that time gave such marvellous charm to the sunsets, was due to a 

 diffraction caused by fine powders, and it became evident that the 

 formidable volcanic explosion of Krakatoa was the prime cause of it." 

 To Cornu we owe some admirable studies upon the conditions for 

 achromatism in the phenomena of interference; a solution of the 

 problem of photometry of polarised light ; and researches on the focal 

 anomalies of diffraction gratings. Shortly after the announcement 

 of the Zeeman phenomenon, Cornu discovered that the line D, under 

 magnetisation in the normal direction, is decomposed into four com- 

 ponents. He also published an elegant experimental method for the 

 investigation of the optical constants of lens systems. He devised 

 the optical lever for the measurement of the curvatures of lenses; 

 and he perfected the Jellett prism for polarimetric work. To him js 



