All space is supposed to be filled with a substance called ether, and that it permeates even solid material. When, for any reason, the natural motion of the molecules of matter is much increased, these molecules have the power of imparting their vibration to the ether in contact with them, and that in turn may produce vibrations in other substances, and if these vibrations come in contact with the nerves of touch, there follows the sensation of warmth or heat. If the vibrations of the ether are still more rapid, when they fall upon the retina, we have the sensation of sight, and we call the agent light. Heat and light, then, are the same. In one instance the vibration is capable of affecting one set of nerves, and in the other, two sets of nerves. The heat-vibration can be discovered by the sense of touch alone, but the light-vibration may be detected both by the eye and the touch. This variation in sensations, when produced by the same cause, may be illustrated as follows: Apply some salt to the tongue, and place some also in a wound, the two sensations are entirely unlike. Again, the vibrations of a body may be so slow that we can discover them by touch, as showing resistance, or so rapid that they are reported to the ear as a shrill sound, or they may be increased so intensely as to evolve heat, and if still more increased in rapidity, affect the eye as light. The spectrum affords us still another illustration of this truth. Pass through a prism a single ray of light, lo, it appears on the screen in all the colors of the rainbow. Nor is this all; _between_ the bright colors, and _beyond_ the violet and the red are invisible lines, and the various parts of the spectrum, although all are produced by the one ray, are capable of creating quite different results. If one should place a delicate thermopile below the red color, it at once reports heat, although the eye sees nothing there. The beautiful colors of the spectrum flash their light into the eye, raise the temperature of the thermometer and affect chemical transformations, while, still more wonderful, the dark lines above the violet, though unseen and not indicated by the thermopile, act upon the sensitized plate of the photographer with decided chemical force. Thus changes in vibrations as to rapidity, length and direction make changes in the resulting sensations.