The latest invention is the photophone. The rare metal selenium
has the property, when acted on by light, of offering more resistance to the passage of electricity than when in dark- ness. Mr. Graham Bell, availing himself of this property, has arranged an apparatus by which a ray of light falling on a selenium cell repeats sounds uttered at the back of the mirror reflecting the ray. The cell is placed "in circuit" with a battery and telephone, and the sounds are distinctly audible at 200 yards. It is believed, though not proved, that they would be audible at any distance to which light can be thrown ; and if so, most important improvements could be effected in helio- graphy, and messages from balloons might in war be trans- mitted with ease without a descent. Mr. Bell has also invented an instrument of extreme interest, though as yet without prac- tical application. " By means of a perforated disc rotating before a fixed perforiated screen, a beam of light was rapidly inter-
rupted. These rapid alternations of light and darkness falling on the selenium produced a musical note in the telephone, the rotating disc itself being absolutely silent," a phenomenon which suggests strange thoughts. Suppose our descendants get an audible message from a star. That cipher would be forth deciphering.
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