394 PROCEEDINGS OF THE AMERICAN ACADEMY. 



same year, the Rumford jNIedal of the American Academy. This 

 is the only case in which both of these awards have been made to 

 the same person. The existence of far longer infra-red ether waves 

 has been proved by Rubens and his students in Berlin, among 

 whom were several Americans, two of these being E. F. Nichols 

 and R. W. Wood, both of whom have received the Rumford JNIedal 

 of this Academy. An indirect method, that of measuring the 

 " rest-strahlen " was chiefly employed. These go down to one- 

 third mm. These longest "heat waves" almost reach the length 

 of the shortest Hertz waves, which are only about 2 mm. long and 

 whose length increases up to thousands of feet as with those used 

 in radio-telegraphy. 



At about the same time that the elder Herschel discovered the 

 existence of infra-red rays, Ritter and Wollaston showed that 

 certain silver salts were acted upon photographically by previously 

 unknown radiations outside of the violet, the ultra-violet rays, 

 and E. Becquerel demonstrated their photographic action in 1842 

 as existing up to 3,400 A. in the solar spectrum. Stokes's discov- 

 ery of fluorescence in 1851 enabled him by this new method to 

 detect ultra-violet radiations of wave-length 3,000 A in sunlight, 

 and up to 1850 A in the light of the electric spark between metal 

 electrodes. In the study of these very highly refrangible rays 

 Stokes was obliged to use jmsms and lenses of quartz on accoimt 

 of the great absorptive power of glass for such radiations. Cornu 

 reached the same limit by photography. 



A most serious obstacle to the further prosecution of researches 

 of this character was the highly absorptive power of atmospheric 

 air for these extreme rays even in thin layers. This set a limit to 

 further advances by known methods. 



^'ictor Schumann, somewhat before 1890, undertook to overcome 

 the difficulty by constructing a vaciuun spectroscope and camera. 

 In this, the rays from an electric discharge e. g., through hydrogen 

 in a vacuum tube with a fluorite window, passed through an 

 inclosed oj)tical train of prism and lenses of fluorite, which sub- 

 stance he had found to be more transparent to higher radiations 

 than quartz. The rays passed through a rarefied atmosphere of 

 hydrogen on their way to the photographic plate instead of through 

 air at the ordiiiarv densitv. Under these conditions it was found 



