PHOTOPHONE (from Gk. Of, phos, light OLovh, idiom', voice), flAnienoNE, or SPECTRO PHONE. Instruments in which radiant energy in the form of light acts to produce sound were de vised by Prof. Alexander Graham Bell (q.v.) in conjunction with Sumner Fainter. The invention of the photophone arose from the employment of illoughy Smith of selenium (q.v.) as a resist ance medium in testing submarine cables. It was found that the electrical resistance of sele nium varied considerably, and the discovery was made that this was due to the action of light, which lessened the resistance. \Verner Siemens tin 1875) produced some extremely sensitive va rieties of selenium, and in some of his experi ments the resistance on exposure to light was found to he only one-fifteenth of what it was in the dark. Professor Bell substituted the tele phone for the galvanometer and used intermit tent Hashes of light to produce a succession of changes in the conductivity of the selenium, cor responding in frequency to the musical vibra tions within the limits of hearing, as it ap peared to him that all the audible effects obtained from electricity could also be produced by varia tions in the light acting on selenium. He per ceived the effect could be produced at the extreme distance at which selenium would respond to the action of a luminous body, and that this distance could be indefinitely increased by the use of a parallel beam of light, so that it would be pos sible to telephone from one place to another with out a conducting wire, were apparatus devised by which variations could be producer] in a parallel !wain of light, corresponding to the variations in the air produced by the voice. After a number of experiments in which the resistance of the sele nium cells was greatly reduced and other diffi culties overcome, a form of photophone was de vised iu which light reflected from a plain mir ror of tlexihle material, such as silvered mica or a microscope cover glass, against the back of which the speaker's voice is directed, was caused to vibrate with a frequency corresponding to that of the diaphragm itself. For reproducing sound at a distance sunlight or any other powerful light may be used. and a large beam is concen trated by means of a lens upon the diaphragm mirror, which after reflection is again rendered parallel by means of another lens. The beam is received at a distant station upon a parabolic re flector. in the focus of which is placed a sensitive selenium cell, connected in a local circuit with a battery and a telephone. The loudest effects were produced by rapidly interrupting the beam of light by a perforated rotating disk, revolving over the face of another perforated disk, with holes corresponding. These experiments led to others with substances other than selenium, and with out the use of telephone or battery. A thin sheet • of hard rubber was held close to the car while a beam of intermittent light was thrown upon it by a lens, the result being the production of a musical note, and this effect was intensified by arranging the hard rubber as a diaphragm and listening through a hearing,-tube. The conclusion
was reached that sounds could lie produced by the action of a variable light from substances of all kinds when in form of thin diaphragms. Subse quently Professor Bell concluded that sonorous ness under the influence of intermittent light was a property of all matter. Various experiments with different fibrous and porous materials, such as cotton-wool, worsted, silks, sponge, lampblack, etc., indicated that lampblack may be substi tuted for selenium in an electrical receiver. lo France, M. Mercardier passed an intermittent beam from an electric lamp through a prism and found a difference in the audible effects in differ ent parts of the spectrum. These experiments were repeated by Professor Bell, who ascertained that 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 continu ous increase in the loudness of the sound was ob served upon moving the receiver gradually from the violet into the ultra-red, corresponding ex actly with the proportion of heat in different parts of the spectrum, and showing that these ef fects are produced by the beat which accompanies light. Beyond this point the sound began to de crease, and then stopped so suddenly that a very slight motion of the receiver made all the differ ence between almost maximum sound and com plete silence. Experiments with various sub stances led to the conclusion that the nature of the rays that produce sonorous effects in different substances depends upon the nature of the sub stances 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. These considerations led Professor Bell to con struct a new instrument for use in speetrnm analysis, which lie termed a `speetrophone.' The eyepiece of a spectroscope was removed and sensi tive substances were placed in the focal point of the instrument behind an opaque diaphragm eontaining a slit. These substances are put in communication with the ear by means of a hear ing-tube, and if we smoke the interior of our spectrophonie receiver and fill the cavity with peroxide of nitrogen gas. we have then a combi nation 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 ab sorption spectrum is to lie investigated. and bands of sound and silence are observed upon exploring the spectrum, the silent positions cor responding to the absorption bands. The ear eannot compete with the eye in the ,•xamination of the visible part of the spectrum; but in the invisible part beyond the red the method is ap plicable. In working in this region of the spec trum, lampblack alone may be used in the spec trophonic receiver. Professor hell stated that the spectrophone was only an adjunct to the spec troscope, but he considered that it would have a wide and independent field of usefulness in the in vestigation of absorption spectra in the ultra-red.