PHILOSOPHICAL SOCIETY OF WASHINGTON. 161 



substances that are exposed to the beam, and that the sounds are in 

 every case due to those rays of the spectrum that are absorbed by the 

 body. 



The Spectrophone. 



Our experiments upon the range of audibility of different sub- 

 stances in the spectrum have led us to the construction of a new 

 instrument for use in spectrum analysis. The eye-piece of a spec- 

 troscope is removed, and sensitive substances are placed in the 

 focal point of the instrument behind an opaque diaphragm con- 

 taining a slit. These substances are put in communication with 

 the ear by means of a hearing-tube, and thus the instrument is 

 converted into a veritable "spectrophone," like that shown in 

 Fig. 13. 



Suppose we smoke the interior of our spectrophone receiver, and 

 fill the cavity with peroxide of nitrogen gas. We have then a 

 combination that gives us good sounds in all parts of the spectrum, 

 (visible and invisible,) except the ultra-violet. Now, pass a rapidly- 

 interrupted beam of light through some substance whose absorption 

 spectrum is to be investigated, and bands of sound and silence are 

 observed upon exploring the spectrum, the silent positions corres- 

 ponding to the absorption bands. Of course, the ear cannot for 

 one moment compete with the eye in the examination of the visible 

 part of the spectrum ; 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, lamp-black alone may be used in the spec- 

 trophonic 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 thermo-pile. 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 the 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. 



Result : Well-marked sounds in every part of the ultra-red. No 

 11 



