Page:Heroes of the telegraph (IA cu31924031222494).djvu/202

 He was not the first to entertain the idea, for in the summer of 1878, one 'L. F. W.,' writing from Kew on June 3 to the scientific journal Nature describes an arrangement of the kind. To Professor Bell, in conjunction with Mr. Summer Tainter, belongs the honour of having, by dint of patient thought and labour, brought the photophone into material existence. By constructing sensitive selenium cells through which the current passed, then directing a powerful beam of light upon them, and occulting it by a rotary screen, he was able to vary the strength of the current in such a manner as to elicit musical tones from the telephone in circuit with the cells. Moreover, by reflecting the beam from a mirror upon the cells, and vibrating the mirror by the action of the voice, he was able to reproduce the spoken words in the telephone. In both cases the only connecting line between the transmitting screen or mirror and the receiving cells and telephone was the ray of light. With this apparatus, which reminds us of the invocation to Apollo in the Martyr of Antioch—

Professor Bell has accomplished the curious feat of speaking along a beam of sunshine 830 feet long. The apparatus consisted of a transmitter with a mouthpiece, conveying the sound of the voice to a silvered diaphragm or mirror, which reflected the vibratory beam through a lens towards the selenium receiver, which was simply a parabolic reflector, in the focus of which was placed the selenium cells connected in circuit with a battery and a pair of telephones, one for each ear. The transmitter was placed in the top of the Franklin schoolhouse, at Washington, and the receiver in the window of Professor Bell's laboratory