RECORDS OF MEETIXGS. 395 



that radiations far above any previously detected were recognized 

 on the photoo-raphic ])late. Schumann also employed an emulsion 

 of his own invention in order to secure better action with such short 

 waves as those he was to deal with, which were strongly absorbed 

 by gelatine. As a result he obtained excellent photographs of 

 hydn)gen lines which he belie\'ed to have a wave length as short 

 as 1,000 A. lie also mapped the lines of various other gases. 

 But manifestly using as he did a prismatic spectrum it could not 

 be certain first that higher radiations were not absorbed by the 

 fluorite and second that the wave-lengths of those which were 

 transmitted by it were correctly estimated. In fact there were 

 serious errors arising from both these causes. 



It was here that ]\Ir. Lyman took up the matter. He 

 avoided vSchumann's difficulty arising from the use of a prism spec- 

 troscope by employing a Rowland concave diffraction grating, 

 which serves at once as a mirror wherewith to focus the rays upon 

 the sensitive plate and a grating to disperse them and which, more- 

 over, allows exact measurement of the wave-length of the photo- 

 graphed lines, while, of course, no prisms or lenses being used the 

 difficulty arising from their absorption does not exist. The grat- 

 ing had 15,028 lines to the inch. He found that a grating was 

 physically suitable for use with these high radiations, and he 

 devised an ingenious two-slit method for determining accurately 

 the wave lengths of all such lines as should be photographed. 

 He also made use of a curved photographic plate for better defini- 

 tion. 



As was the case in Schumann's research the whole apparatus 

 was placed in a vacuum chamber. This was filled with hydrogen 

 gas scrupulously purified and rarefied to a density of about 1 /oOO 

 of an atmosphere. 



To avoid difficulty from the absorption of the radiations from 

 the electric spark which would be exercised even by a fluorite 

 window in the vacuum tube in which the electrical spark was 

 produced this opened directly into the chamber containing the spec- 

 troscope so that the radiations passed only through rarefied h>-dro- 

 gen before and after they fell upon the grating and reached the 

 camera. By this device Professor Lyman was able to photograph 

 and measure rays whose wave length Avas only 905 A, and later 



