Alexander Graham BELL, Ph.D., 
"On the Production and Reproduction of Sound by Light", 
American Journal of Sciences

Third Series, vol. XX, n°118, Oct. 1880, pp. 305- 324.

[Read before the American Association for the Advancement 
of Science, in Boston, August 27, 1880]

Experiments to ascertain the nature of the rays that affect selenium.

    We have made experiments with the object of ascertaining the nature of the rays that affect selenium. For this purpose we have placed in the path of an intermittent beam various absorbing substances.

    Prof. Cross has been kind enough to give his assistance in conducting these experiments.

    When a solution of alum, or bisulphide of carbon, is employed, the loudness of the sound produced by the intermittent beam is very slightly diminished, but a solution of iodine in bisulphide of carbon cuts off most, but not all, of the audible effect. Even an apparently opaque sheet of hard rubber does not entirely do this.

    This observation, which was first made in Washington, D. C., by Mr. Tainter and myself, is so curious and. suggestive that I give in full the arrangement for studying the effect.

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    When a sheet of hard rubber, A, was held as shown in the diagram (fig. 9) the rotation of the disc or wheel B interrupted what was then an invisible beam, which passed over a space of several meters before it reached the lens C, which finally concentrated it upon the selenium cell, D.

    A faint but perfectly perceptible musical tone was heard from the telephone connected with the selenium that could be interrupted at will by placing the hand in the path of the invisible beam.

    It would be premature without further experiments to speculate too much concerning the nature of these invisible rays; but it is difficult to believe that they can be beat rays, as the effect is produced through two sheets of hard rubber having  between them a saturated solution of alum.

    Although effects are produced, as above shown, by forms of radiant energy which are invisible, we have named the apparatus for the production and reproduction of sounds in this way "the Photophone," because an ordinary beam of light contains the rays which are operative.

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