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In May, 1878, Mr. Alexander Graham Bell, will known in connection with the telephone, announced before a scientific society in London Ins belief that it would be possible to hear a shadow by interrupting the action of light upon sale. nium. At the recent meeting of the American Science Association in Boston, Mr. Bell read a paper describing at length his experiments in the production and reproduction of sound by light, and the invention by Mr. Sumner Tainter and himself of an instrument for the purpose. The infiuence of light upon the electric conducting power of selenium is well known. Mr. Bell found the electric resistance of same selenium cells of peraHar cn- struction only one-fifteenth as much in the light as in the dark. It occurred to him that all the audible effects obtained in the telephone by variation of the electric current by sound waves, could also be produced by variation of light acting upon selenium; and that with suitable transmitting and receiving apparatus voices mW be conveyed without a wire along a line of light. The fundamental idea on which rests the possibility of producing speech by the action of light is the conception of what-Mr. Bell terms an undulatory beam of light in contradistinction to an interrupted beam; meaning by the former a beam that shines continuously, but is ssiAject; to rapid changes of intensity. The apparatus used to give the required undulatory character to light consists of a fiexible mirror of silvered mica or thin glass. The speaker's voice is directed against the back of this mirror, as against the diaphragm of a telephony and the light refiected from it is thereby thrown into corre- sp0nding undulations. In his experiments, chiefiy with sunlight, Mr. Bell concentrates upon the diaphragm mirror a beam of light, which, after refiection, is again rendered parallel by means of another lens. The beam proceeding from the transmitter is received at a distant station upon a parabolic reflector, in the center of which is a sensitive selenium cell connected in a local circuit with a battery and telephone. In a recent experiment. Mr. Bell's associate operated the transmitting instrumra which was placed on the top of the Franklin school house, in Washington, about eight hundred feet distant from the receiver, placed in a window of Mr. Bell's laboratory. Through this distance messages were distinctly conveyed by means of' light. In his laboratory experiments Mr. Bell fnds that articulate speech can be transmitted and reproduced by the light of an oxyhydrogen lamp, and even by the light of a kerosene lamp. The rapid interruption of the beam of light by a perforated disk gives rise to musical tones, siren fasbion. With this apparatus silent motion produces sound, loud musical tones being emitted from the receiver when no sound is made at the transmitter . The importance of these investigations it is impossible now to estimate. That the photopbone can practically take the place of the telephone is not likely, though it is likely to work radical changes in military and other signaling operations. The heliograph, which has proved so useful in recent campaigns in the Afghau country and elsew here, can now be made to talk orally yet silently over the heads of an en emy or acr oss im passable stream s or other low barriers. For rapid communication between distant exploring or surveying stations, tbe photophone also promises to be serviceable. Another result of Mr. Bell's researche, in this connection is the discovery that many other substances are sensitive to light. He has found this property . gold,SI'l vel, pl at 'mum, iron, steel, brass, copper, z . c, I ead ,antImony, G ennan sivel, Jenkins' metal, Babbitt's metal, ivory, celluloid, gutta i percha, hard rubber, soft vulcanized rubber. paper, parchment, wood, mica, and silvered glass. The only substance s found insensible to light are carbon and thin microscopic glass.
