PRODUCTION OF SOUND BY RADIANT ENERGY. 333 



the eye is useless, the ear is 

 invaluable. In working in 

 this region of the spectrum, 

 lampblack alone may be used 

 in the spectrophonic receiver. 

 Indeed, the sounds produced 

 by this substance in the ultra- 

 red are so well marked as to 

 constitute our instrument a 

 most reliable and convenient 

 substitute for the thermopile. 

 A few experiments that have 

 been made may be interest- 



ing. 



1. The interrupted beam 

 was filtered through a sat- 

 urated solution of alum. 



Result : The rans^e of au- 

 dibility in the ultra-red was 

 slightly reduced by the ab- 

 sorption of a narrow band of 

 the rays of lowest refran- 

 gibility. The sounds in the 

 visible part of the si^ectrum 

 seemed to be unaffected. 



2. A thin sheet of hard 

 rubber was interposed in the 

 path of the beam. 



Result : Well-marked 

 sounds in every part of the 

 ultra-red. Ko sounds in the 

 visible part of the spectrum, 

 excepting the extreme half 

 of the red. 



These experiments reveal 

 the cause of the curious fact 

 alluded to in my paper read 

 before the American Asso- 

 ciation last August that 

 sounds were heard from se- 

 lenium when the beam was 

 filtered through both hard 

 rubber and alum at the same 

 time. (See table of results in 

 Fig. 14.) 

 3, A solution of ammonia- sulphate of coj^per was tried. 



