, r >2^ On the Production of Sound by Radiant Energy. 



bands. Of course, the ear cannot for one moment compete 

 with the eye in the examination of the visible part of the spec- 

 trum ; but in the invisible part beyond the red, where the eye 

 is useless, the ear is invaluable. In working in this region of 

 the spectrum, lampblack alone may be used in the spectro- 

 phonic receiver. Indeed the sounds produced by this sub- 

 stance 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 interesting. 



(1) The interrupted beam was filtered through a saturated 

 solution of alum. 



Result. The range of audibility in the ultra-red was slightly 

 reduced by rhe absorption of a narrow band of the rays of 

 lowest refrangibility. The sounds in the visible part of the 

 spectrum seemed to be unaffected. 



(2) A thin sheet of hard rubber was interposed in the path 

 of the beam. 



Residt. Well-marked sounds in every part of the ultra-red. 

 No sounds in the visible part of the spectrum, excepting the 

 extreme half of the red. 



These experiments reveal the cause of the curious fact al- 

 luded to in my paper read before the American Association 

 last August — that sounds were heard from selenium 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 copper was tried. 

 Result. When placed in the path of the beam the spectrum 



disappeared, with the exception of the blue and violet end. To 

 the eye the spectrum was thus reduced to a single broad band 

 of blue-violet light. To the ear, however, the spectrum re- 

 vealed itself as two bands of sound with a broad space of 

 silence between. The invisible rays transmitted constituted 

 a narrow band just outside the red. 



I think I have said enough to convince you of the value of 

 this new method of examination ; but I do not wish you to 

 understand that we look upon our results as by any means 

 complete. It is often more interesting to observe the first 

 totterings of a child than to watch the firm tread of a full- 

 grown man ; and I feel that our first footsteps in this new field 

 of science may have more of interest to you than the fuller 

 results of mature research. This must be my excuse for having 

 dwelt so long upon the details of incomplete experiments. 



I recognize the fact that the spectrophone must ever remain 

 a mere adjunct to the spectroscope ; but I anticipate that it has 

 a wide and independent field of usefulness in the investigation 

 of absorption-spectra in the ultra-red. 



