Light
3 of the 7 encyclopedias on this shelf carry an entry for Light. Both are reproduced below, so you can see where they agree and where they differ.
Collier's New Encyclopedia (1921)
the natural agent which, by ent which, by acting on the retina, excites in us the in us the sensation of vision. Two leading hypoleading hypotheses regarding its nature have been have been formed: the one, the omission or corpusomission ot coгpиbcular theory, which, though supported by supported by the great name of Sir Isaac Newton, has Newton, has been abandoned, and the other, the unthe other the un dulatory theory, which now obtains. hich now obtains. The latter assumes the existence everythe existen everywhere through the universe, or the portion of it with which we are connected, of an extremely subtle elastic medium, le elastic med called luminiferous ether, the undulations Ther, the undulations of which constitute light, and when they ht, and when they impinge on the retina produce vision. The particles agitated are not transagitate mitted but only the disturbance. The disturban movements are held to be strictly analogous to the undulations of the atmosphere which produce and convey sound; or, as the word undulation imports, those of the ocean in producing at Barnegat, erected in 1903, provided Several methods of calculating waves. the velocity with which light is transmitted are known. By one, the size of the minute circle through which the aberration of light makes stars apparently revolve is carefully noted, and the relative proportion of the earth's velocity in her orbit to that of light arriving from the stars ascertained. The result is that light is found to move about a hundred thousand times as fast as the earth, which gives the velocity about 186,000 miles per second. By another, observation is made of the time in which light actually arrives at the eye from one of Jupiter's satellites at the commencement or the close of an occultation as compared with their calculated times. It is found that 8' 18" are required for light to travel over half the earth's orbit, which gives, as in the former case, about 186,000 miles per second for its velocity. ity. The velocity is also measured directly, by two instrumental methods devised respectively by Foucault and results. The Fizeau, with the same great sources of light are the sun, the fixed stars (other suns), bodies in a state of ignition, electricity, etc. The bodies sending forth rays or pencils of light are called luminous; those through which it passes easily, transparent or diaphanous; those through which it passes less easily, translucent; and those through which it cannot pass at all, opaque. When a ray meets the surface of a body, it may be refracted and decomposed or reflected. When it encounters an opaque body it casts a shadow. Admitted into a dark chamber through a small aperture to fall on a screen, the rays make images of external objects reversed. The illuminating power on any surface is inversely as the square of the distance from the source of light. This may be measured by a photometer. Light may be defracted, it may be polarized. An abundant supply of it is essential to the healthy growth of man, the inferior animals, and plants.
Aiton's Encyclopedia (1910)
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
An Encyclopaedia of Occultism (1920)
Spiritualistic Journal. (See Spiritualism .) Lignite is a beautiful stone, like glass; being hung about a child it preserves it from witchcraft, and if bound on the forehead it stops the bleeding of the nose, restores the loss of senses, and helps to foretell future events.