The Photophone

Abstract

EARLY in the summer of the current year it was announced in the columns of NATURE that Prof. Graham Bell had made a discovery which, for scientific interest, would rival the telephone and phonograph, and that he had deposited in a sealed packet, in the custody of the Smithsonian Institute, the first results of his new researches; that announcement has now received its due fulfilment in the lecture by Prof. Bell to the American Association for the Advancement of Science, on Selenium and the Photophone, which will be found on another page. In spite of those who ingenuously attempted at the time of our announcement to forestall Prof. Bell and to discredit the idea that he had done anything new, the discovery, which he has now published, is a startling novelty. The problem which he has attacked is that of the transmission of speech, not by wires, electricity, or any mechanical medium, but by the agency of light. The instrument which embodies the solution of this principle he has named the Photophone. It bears the same relation to the telephone as the heliograph bears to the telegraph. You speak to a transmitting instrument, which flashes the vibrations along a beam of light to a distant station, where a receiving instrument reconverts the light into audible speech. As in the case of that exquisite instrument, the telephone, so in the case of the photophone, the means to accomplish this end are of the most ridiculous simplicity. The transmitter consists of a plane silvered mirror of thin glass or mica. Against the back of this flexible mirror the speaker's voice is directed; a powerful beam of light is caught by a lens from the sun and directed upon the mirror, so as to be reflected straight to the distant station. This beam of light is thrown by the speaker's voice into corresponding vibrations. At the distant station the beam is received by another mirror, and concentrated upon a simple disk of hard rubber fixed as a diaphragm across the end of a hearing-tube. The intermittent rays throw the disk into vibration in a way not yet explained, yet with sufficient power to produce an audible result, thus reproducing the very tones of the speaker. Other receivers may be used, in which the variation in electrical resistance of selenium under varying illumination is the essential principle. The experimental details have been worked out by Prof. Bell in conjunction with Mr. Sumner Tainter. They have discovered that other substances beside hard rubber, gold, selenium, silver, iron, paper, and notably antimony, are similarly sensitive to light. This singular production of mechanical vibrations by rays of light is even more mysterious than the production of vibrations in iron and steel by changes of magnetisation. It was indeed this latter fact which led the discoverers to suspect the analogous phenomenon of photophonic sensibility in selenium and in other substances. Hitherto, in consequence of the mere optical difficulties of managing the beam of light, the distance to which sounds have been actually transmitted by the Photophone is less than a quarter of a mile, but there is no reason to doubt that the method can be applied to much greater distances, and that sounds can be transmitted from one station to another wherever a beam of light can be flashed; hence we may expect the slow spelling, out of words in the flashing signals of the heliograph to be superseded by the more expeditious whispers of the Photophone.