“Mr. Willoughby Smith, an engineer engaged in the laying of submarine cables, had devised a system of testing and signalling during their submersion. For this system, in 1872, it occurred to him that he might employ crystalline selenium, on account of its high resistance, at the shore end of a cable. On experiment the selenium was found to have all the resistance required; some of the bars displayed a resistance of 1400 megohms, as much as would be offered by a telegraph wire long enough to reach from the earth to the sun. But this resistance was found to be extremely variable; the reason was disclosed when Mr. May, an assistant, observed that the resistance of selenium is less in light than in darkness. This discovery created widespread interest throughout the world. Among the investigators who at once turned their attention to the subject was Professor W. G. Adams of King’s College, London, who proved that the action on selenium is chiefly due to the luminous rays of the spectrum, the ultra-red and ultra-violet rays having little or no effect. Dr. Werner Siemens, the eminent German physicist, produced a variety of selenium fifteen times more conductive in sunlight than in darkness. This extraordinary sensitiveness was brought about by heating for some hours at a temperature of 210° C., followed by extremely slow cooling. [Illustration: Telephones receiving sounds through a beam of light.] The Telephone Brought in. “Observations concerning the effect of light upon the conductivity of selenium had employed the galvanometer solely; it occurred to me that the telephone, from its extreme sensitiveness, might be substituted with advantage. On consideration I saw that the experiments could not be conducted in the ordinary way with continuous light, for a good reason: the law of audibility of the telephone is precisely analogous to the law of electrical induction. No effect is produced during the passage of a continuous and steady current. It is only at the moment of change from a stronger to a weaker state, or, vice versa, that any audible effect is produced; this effect is exactly proportional to the amount of variation in the current. It was, therefore, evident that the telephone could only respond to the effect produced in selenium at the moment of change from light towards darkness, or _vice versa_, and that it would be advisable to intermit the light with great rapidity so as to produce a succession of changes in the conductivity of the selenium corresponding in frequency to musical vibrations within the limits of the sense of hearing. For I had often noticed that currents of electricity, so feeble as hardly to produce any audible effects from a telephone when the circuit was simply opened and closed, caused very perceptible musical sounds when the circuit was rapidly interrupted; and that the higher the pitch of the sound the more audible was its effect. I was much struck by the idea of producing sound in this way by the action of light.