This remarkable invention is own sister to the telephone. It means, to talk by light. The idea upon which it is founded is this:

Certain substances are sensitive to light, and change their electrical condition according to the amount of light that falls upon them. To understand this, you may observe that colored cloths fade in the sun, and certain chemicals change their color in a beam of light, as in taking a photograph. This is called the actinic effect of light. This is a new fact in nature, and upon it is founded the new apparatus for talking by light.

The apparatus consists, first, of a transmitter for causing the sound of the voice to affect a beam of sunlight. This is a thin diaphragm of silvered mica arranged somewhat like a diaphragm of a telephone. A powerful beam of sunlight is directed upon the front of this, and is reflected through two lenses to the receiving-station, which may be several hundred feet, or metres, away. The operator stands behind the mirror, and speaks against the back of it. At the receiving-station is a reflector arranged to receive the beam of light and concentrate it upon a curious sub-stance discovered a few years ago, and called celenium, and connected in a peculiar manner with a telephone.

Now, if the operator speaks behind the mica mirror, the person holding the telephone to his ear hears every word that is said. To trace the curious changes the sound goes through, from one operator to the other, we must observe that the vibrations of the air move the mirror, and cause the beam of reflected light to vibrate. The vibrations of the light affect the electrical condition of the telephone; the electrical vibrations are transformed in the telephone back again into sounds. This truly wonderful invention is so new that it is impossible to say what may result from it. This much has, however, been proved: the sound of the human voice and musical notes may be sent to a distance by means of a beam of sunlight or by the light of a lamp.