Photophone. It was discovered by Willoughby Smith in 1875 that when selenium forms part of a closed circuit, its resistance changes with varying illumination. Vitreous selenium has an enormously high resistance, but in its crystalline condition its conductivity is much increased, and it is then most influenced by light. Professor Adams showed that the change of resistance was proportionally the square root of the illumination. Upon this property depends the action of the photophone, which may be said to transmit sound along a beam of light. The person whose speech is to be transmitted causes, by the vibrations of his voice, a light mirror to be thrown into motion. The mirror reflects a beam of light to a distance, where it falls upon a piece of selenium connected in circuit with a battery and telephone. The mirror, in consequence of its vibrations, sends a varying amount of light to the selenium. Hence the resistance of the selenium is altered, and the variation of the current which is thereby caused reproduces the sounds in the telephone. The beam of light is originally sent through a lens, at whose focus the light mirror is placed; at the receiving end it is reflected from a parabolic mirror on to the selenium which is placed at the focus. In this way, greater variation of the light is obtained for the same amount of vibration. It has also been found that selenium will give out a sound without the aid of battery and telephone when a beam of intermittent light is thrown upon it. Later experiments have shown that selenium is not alone in this property. Tellurium, antimony, gold, silver, carbon, parchment, hard rubber, and some kinds of wood are also electrically sensitive to light.