396 PROCEEDINGS OF THE AMERICAX ACADE:\IY. 



those of wave length 600 A. He showed that most serious absorp- 

 tion occurred with Schumann's optical train, so that no rays shorter 

 than 1,267 A were transmitted, which was therefore Schumann's 

 limit. He mapped with great care the rays from the spark in a 

 hydrogen vacuum tube, recording many hundred lines. He also 

 studied the radiations in the Schumann region produced by the 

 electric spark taken between electrodes of other substances, such 

 as aluminium and the alkaline earths, and likewise the absorption 

 of certain solids used for optical purposes and of various gases. 

 It was with one of the latter, helium, that the extreme short wave 

 length, 600 A, was reached. In the progress of the investigations 

 various new lines were measured which were of importance as 

 verifying theoretical prediction regarding spectral series. Of 

 course, in all these last researches the spark had to be taken within 

 a vacuum tube with a fluorite window, so that no radiations above 

 Schumann's limit or thereabouts could be reached. 



There is a great region beyond these ultra-violet radiations 

 studied by Schumann and Lyman, doubtless occupied by rays which 

 have not yet been detected although searched for, but far above 

 this limit are found the X-rays, long believed to be ether waves, 

 and more recently proved to be such by the actual measure of their 

 length by the Braggs, father and son. Their wave length is 1 A, 

 more or less, according to conditions. This is the extreme limit of 

 measured ether waves, but the so-called "Gamma rays," emitted 

 by radioactive substances, have a waxe length estimated at perhaps 

 1/10 A. 



The President presented the medals to Professor Lyman, who 

 expressed his thanks, and then addressed the Academy on " Flash 

 and Sound Hanging with the American Forces in France. " 



The meeting then adjourned. 



One thousand and eighty-second Meeting. 



May U, 1919.— Annual ^^Ieeting. 



The Academy met at its House. 



The President in the Chair. 



There were thirty-six Fellows antl one guest present. 



