Light

a form of wave motion closely akin to heat and electricity. It is thought that light waves are between those of heat and electricity in point of length and frequency. Light waves are shorter and quicker than heat waves. They are slower and less frequent than those of electricity. It is certain that heat, light, and electricity may be converted, the one into the others. The great source of light is the sun. Light is thought to be communicated by crosswise, that is to say, transverse vibrations of particles of a little understood medium called ether. This ether fills all space. Under uniform conditions, light travels in straight lines. It is bent out of its course, however, or refracted in passing obliquely from rare to a denser medium, as from air to water, and again in passing from a dense to a rarer medium, as from water to air. In each case, the rays of light are bent toward a line perpendicular to the refracting surface. Oblique rays are refracted also in passing from air into glass or from glass into air. Advantage is taken of this fact to construct lenses. A lens has the power, according to its shape, of bringing rays to a focus, or of causing them to diverge. It has been computed that light travels through space at the rate of 186,330 miles per second. Light makes the journey from the sun in 499 seconds, a trifle over eight minutes. Light is necessary to animal and vegetable life. No matter how favorable other conditions may be, ordinary plants and animals cannot thrive in darkness. See Heat; Electricity; Gas; Newton; Argand; Faraday; Candle; Lamp; Fire; Sun; Eclipse; Spectrum Analysis; Color; Green; Blue; Red; Yellow; Rainbow; Lens; Microscope; Telescope; Photograph; Mirage; Finsen; X-Ray; Eye; Blindness