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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--'Lord of the speaking lyre, That with a touch of fire Strik'st music which delays the charmed spheres.'
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 in L Street. 'It was impossible,' says the inventor, 'to converse by word of mouth across that distance; and while I was observing Mr. Tainter, on the top of the schoolhouse, almost blinded by the light which was coming in at the window of my laboratory, and vainly trying to understand the gestures he was making to me at that great distance, the thought occurred to me to listen to the telephones connected with the selenium receiver. Mr. Tainter saw me disappear from the window, and at once spoke to the transmitter. I heard him distinctly say, "Mr. Bell, if you hear what I say, come to the window and wave your hat! " It is needless to say with what gusto I obeyed.'
The spectroscope has demonstrated the truth of the poet, who said that 'light is the voice of the stars,' and we have it on the authority of Professor Bell and M. Janssen, the celebrated astronomer, that the changing brightness of the photosphere, as produced by solar hurricanes, has produced a feeble echo in the photophone.
Pursuing these researches, Professor Bell discovered that not only the selenium cell, but simple discs of wood, glass, metal, ivory, india-rubber, and so on, yielded a distinct note when the intermittent ray of light fell upon them. Crystals of sulphate of copper, chips of pine, and even tobacco-smoke, in a test-tube held before the beam, emitted a musical tone. With a thin disc of vulcanite as receiver, the dark heat rays which pass through an opaque screen were found to yield a note.
Even the outer ear is itself a receiver, for when the intermittent beam is focussed in the cavity a faint musical tone is heard.
Another research of Professor Bell was that in which he undertook to localise the assassin's bullet in the body of the lamented President Garfield. In 1879 Professor Hughes brought out his beautiful induction balance, and the following year Professor Bell, who had already worked in the same field, consulted him by telegraph as to the best mode of applying the balance to determining the place of the bullet, which had hitherto escaped the probes of the President's physicians. Professor Hughes advised him by telegraph, and with this and other assistance an apparatus was devised which indicated the locality of the ball. A full account of his experiments was given in a paper read before the American Association for the Advancement of Science in August, 1882.
Professor Bell continues to reside in the United States, of which he is a naturalised citizen. He is married to a daughter of Mr. Gardiner G.
Hubbard, who in 1860, when she was four years of age, lost her hearing by an illness, but has learned to converse by the Horace-Mann system of watching the lips. Both he and his father-in-law (who had a pecuniary interest in his patents) have made princely fortunes by the introduction of the telephone.