The Photophone consists of a transmitter for receiving the voice and conveying it along the beam of light, and a receiver for taking the light and converting it into sound—the receiver being the telephone. There is a small mirror (silvered mica has been used) suspended freely for vibration. A lens is used to transmit to this the beam of light, and this beam is again reflected by another lens to the receiver, which consists of a reflector which has a cell of selenium in its focus, connected, as already stated, with the telephone and battery. The speaker stands behind the mirror, and the sound of his voice against the reverse side makes it vibrate in unison with the sounds uttered. The movements cause a quivering in the reflected beam, and this in its changing intensity acts on the selenium, which changes its resistance accordingly, and through the telephone gives forth a sound! This is the apparently complicated but really simple, and at the same time wonderful, invention of Professor Bell. By the Photophone not only sounds but _movements_ can be converted into sound; even the _burning of a candle can be heard_! The Photophone is still capable of improvement, and has not as yet arrived at its full development, for it is stated it can be made quite independent of a battery or telephone. There are many phenomena connected with the _Polarization of Light_. This requires some notice at our hands. We know that a ray of ordinary light is supposed to be caused by vibrations of the highly attenuated medium, æther. These vibrations occur across the direction of the ray; but when they occur only _in one plane_ the light is said to be “polarized.” Polarization means possessing poles (like a magnet); the polarized rays have “sides,” as Newton said, or, as explained by Dr. Whewell, “opposite properties in opposite directions, so exactly equal as to be capable of accurately neutralizing each other.” There are some crystals which possess the property of “double refraction,” and thus a ray of common light passing through such a crystal is divided into two _polarized_ rays, taking different directions. One is refracted according to the usual laws of refraction; the other is not, and the planes of polarization are at right angles. It is difficult within the limits of this chapter to explain the whole theory of Polarization. In order to account for certain phenomena in optics, philosophers have assumed that rays possess polarity; and polarized light is light which has had the property of Polarization conferred upon it by reflection, refraction, or absorption. Common light has been compared to a round ruler, and polarized light to a flat ribbon. Huygens found out, when engaged upon the investigation of double refraction, that the rays of light, divided by passing through a crystal (a rhomb) of Iceland spar, possessed certain qualities. When he passed them through a second rhomb, he found that the brightness, relatively, of the rays depended