LIGHT (Fr., LumiPre ; Ger., Licht) Light is generally considered to be due to minute undulatory waves in the ether that pervades all space. The molecules of any luminous body are in a state of rapid vibration, this movement being communicated to the adjacent ether particles and transmitted by a wave-like motion to the eye, the impact of the waves on the retina giving the sensation of light. In free space light moves with the velocity of i 86,000 miles per second. It travels invariably in a straight line, in a medium of uniform density. Light falling on a mirror or other polished surface is either wholly or partially reflected, at an angle equal to that at which it strikes the reflecting surface. (See " Reflec tion.") White light is not homogeneous, but a mixture of various colours, of different wave lengths. The colours of objects in Nature are not inherent in themselves, but due simply to their absorbing certain rays and transmitting others. (See Colour.") If a beam of light is caused to pass through a small opening or slit and to fall upon a prism, it is separated into its constituent colours. (See " Dispersion.") Besides the visible rays of the spectrum so obtained there are others that are invisible. Those beyond the violet end of the spectrum are known as the ultra-violet rays, and are of great actinic power ' • at the opposite end are the infra-red rays. If a prismatic spectrum is thrown on a sheet of white paper in a darkened room, and the region beyond the violet is painted with a solution of quinine sulphate, it will at once be illuminated with a violet light. It is thus seen that certain substances have the power of reflecting or emitting rays quite different from any that are originally thrown upon them. (See " Fluorescence.") When d ray of light passes obliquely from one trans parent medium into another of different density it is bent aside from its course. (See " Refrac
tion.") If light falls at an angle on a very thin film or plate it is partly reflected at the first surface, while part passes through, undergoes refraction, and is reflected back from the second surface in a direction parallel to the portion first reflected. The two sets of light waves will interfere with each other at certain points, in such a manner that the waves appertaining to one or more of the colours will be extinguished, so that the reflected light appears of a colour complementary to the missing rays. The particular rays extinguished will vary according to the obliquity of the incident ray and the thickness of the film or plate, so that if the film is of varying thickness a number of colours will be seen. It is in this way that the play of colour in a soap-bubble is caused, as also the phenomenon known as Newton's rings, which is turned to account in testing the figuring of lenses. (See " Interference of Light.") When light from a bright point or slit passes through a minute aperture it emerges as a series of coloured bands ; the same effect occurs when light is reflected from a surface covered with a number of very fine indentations or scratches. (See " Diffraction.") Certain crystals, such as Iceland spar, have the property of dividing a ray of light into two distinct refracted parts, one obeying the ordinary law of refraction, while the other has a peculiar law of its own. (See " Polarisation.") Many substances continue to give out light in darkness after removal from the exciting source of light, a phenomenon known as phosphorescence (which see). (See also " Absorption " and " Spectrum.")