44 



NA TURE 



{_May 12, 1 88 1 



vised an ipparatus for andyin.; the effects, which he described 

 to me upon n.y return from Europe. [The apparatus has since 

 been constructed, and Mr. Bell gave a detailed description 

 of it.] 



The meaning we have uniformly attached to the words " pho- 

 to phone " and ' ' light " will be obvious from the following passai^e 

 quoted from my Boston paper : — 



"Although effects are produced as above shown by forms of 

 radiant energy, which are invisible, we have named the appara- 

 tus for the production and reproduction of sound in this way the 

 'photophone,' becau e an ordinary beam of light contains the 

 rays which are operative." 



To avoid in future any misunderstandings upon this point we 

 have decided to adopt the term ^^ radiophone" proposed by M. 

 Mercadier as a general term signifying an apparatus for the pro- 

 duction of sound by any form of radiant energy, limiting the 

 words Ihcnuop/ioiw, pliotophone, and aclinophone to apparatus for 

 the production of sound by thermal, luminous, or actinic rays 

 respectively. M. Mercadier, in the course of his researches in 

 radiophony, passed au intermittent beam from an electric lamp 

 through a prism, and then examined the audible effects produced 

 in different parts of the spectrum ( Coniptes rendtis, December 6, 

 l88o). We have repeated this ex; erinient, u-ing the sun as our 

 source of radiation, and have obtained results somewhat different 

 from those noted by M. Mercadier. A beam of sunlight was 

 rellected from a heliostat through an achromatic lens, so as to 

 form an image of the sun upon the slit. The beam then passed 

 through another achromatic lens and through a bisulphide of 

 carbon prism, forming a spectnira of great intensity, which, 

 when fccvissed upon a screen, was found to be sufficiently pure 

 to show the principal absorption lines of the solar spectrum. 

 The disli-interrupter was then turned with ."-ufficient rapidity to 

 produce from five to six hundred interruptions of the light per 

 second, and the spectrum was explored «ith the receiver, which 

 was so arranged that the lamp-black surface exposed was limited 

 by a slit, as shown. 



Under these circumstances sounds were obtained in every 

 part of the visible spectrum, excepting the extreme half of the 

 violet, as well as in the ultra-red. A continuous increase in the 

 loudness of the sound was observed upon moving the receiver 

 gradually from the violet into the ultra-red. The point of maxi- 

 mum sound lay very far out in the ultra-red. Beyond this 

 point the sound began to decrease, and then stopped so sud- 

 denly that a very slight motion of the receiver made all the 

 ditference between almost maximum sound and complete silence. 



2. The lamp-blacked wire gauze was then removed and the 

 interior of the receiver was filled with red worsted. Upon 

 exploring the sjectrum as before, entirely difterent results were 

 obtained. The maximum effect was produced in the green at 

 that part where the red worsted appeared to be black. On 

 either side of this point the sound gradually died away, be- 

 coming inaudible on the one side in the middle of the indigo, 

 and on the other at a short distance outside the edge of the red. 



3. Upon substituting green silk for red worsted the limits of 

 audition appeared to be the middle of the blue and a point a 

 short distance out in the ultra-red. Maxiumm in the red. 



4. Some hard-rubber shavings were no\v placed in the re- 

 ceiver. The limits of auditiility appeared to be on the one hand 

 tlie junction of the green and blue, and on the other the outside 

 edge of the red. Maximum in the yellow. Mr. Tainter thought 

 he could hear a little way into the ultra-red, and to his ear the 

 maximum was about the junction of the red and orange. 



5. A test-tube containing the vapour of sulphuric ether was 

 then substituted for the receiver. Commencing at the violet 

 end, the test-tube was gradually moved doun the spectrum and 

 out into the ultra-red without audible effect, but when a certain 

 point far out in the ultra-red was reached a distinct musical tone 

 suddenly made its nppearance, which di^^appeared as suddenly 

 on moving the test-tube a very little further on. 



6. Upon explori)ig the spectrum with a test-tube containing 

 the vapour of iodine the limits of audibility appeared to be the 

 middle of the red and the junction of the blue and indigo. 

 Maximum in the green. 



7. A tC- t-tube containing peroxide of nitrogen was substituted 

 for that containing icdine. Distinct sounds wtre obtained in all 

 parts of the visible spectrum, but no sounds were observed in 

 the ultra-red. 



The maximum effect seemed to me to be in the blue. The 

 ' ounds were well-marked in all parts of the violet, and I even 

 fancied that the audible effect extended a little way into the 



ultra-violet, but of this I cannot be certain. Upon examining 

 the absorpiion-specfruua of peroxide of nitrogen it was at once 

 observed that the maximum sound was produced in that part of 

 the spectrum where the greatest number of absorption lines 

 made their appearance. 



8. The spectium was now explored by a selenium cell, and 

 the audible effects were observed by means of a telephone in 

 the same galvanic circuit with the cell. The maximum effect 

 was produced in the red. The audible effect extended a little 

 way into the ultra-red on the c ne hand and up as high as the 

 middle of the violet on the other. 



Although the experiments so far made can only be considered 

 as prelimmary to others of a more refined nature, I think we 

 are warranted in concluding that the nature of the rays that 

 produce sonorous effects in different substances depends upon 

 the nature of the 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. 



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 if a 

 spectroscope is removed, and .sensitive substances are placed in 

 the focal point of the instrument behind an opaque diaphragm 

 containing a slit. These substances are put in communication 

 witli the ear by n eans of a hearing-tube, and tlms the instrument 

 is C' nverted into a veritable ''spectrophone." 



Suppose we smoke the interior of our spectrophonic receiver, 

 and fill the cavity \\ ith 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 corresponding to the absorption loands. Of 

 course the ear cannot for one moment compete with the eye in 

 the examination if the visible part of the spectrum ; but in the 

 invisible part beyond the red, where the eye is useless, the enr is 

 invaluable. In working in this region of the spectrum lamp- 

 black 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 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 sounds in the visible part of the spectrum, excepting the 

 extreme half of the red. 



Thee experiments reveal the cause of the curious fact alluded 

 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. 



3. A solution of ammonia-sulphate of copper was tried. 

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



disappeared, w ith the exception of the blue and vi' let end. To 

 the eye the spectrum was thus reduced to a single bir ad band of 

 blue-violet light. To the ear however the spectrum revealed 

 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 under- 

 stand that VI e look upon our results as by any means complete. 

 It is often more interesting to ob erve 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 mu t be my excuse for having dwelt so long upon the 

 details of incomi Icte experiments. 



I lecognise 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. 



