Page:The New International Encyclopædia 1st ed. v. 12.djvu/277

LIGHT. 251 LIGHT. dispersion is due to the fact that ether-waves of dillerciil wave-length have (lillcient indices of re- fraction for any one material substance; that is, they have dip'erent velocities in it. As a rule the velocity decreases, i.e. the index of refrac- tion increases as the wave-length decreases. This is called regular dispersion. There ai'e, how- ever, many substances for which this is not true ; for instance, in f uchsine the indices of lefraction of the red and yellow waves are great- er than these for the blue and violet. If a spectrum of white liglit is formed on a screen by means of a slit, a prism of glass, and a lens, the colors will be arranged in the order red, j'ellow, green, blue, violet — the red being deviated least, i.e. having the least index of refraction. If, however, a prism of fuchsine is used in place of the glass one, the order of colors will be blue, violet, red, yellow — the green being absent. Fuchsine is a sub.stance having a green 'surface- color' (see Color), and so this color is absent in the transmitted light. The resulting spectrum produced by a fuchsine prism is then exactly as if the ordinary spectrum had been cut in two by the removal of the green and the two halves dis- placed over each other. In other words, with bodies possessing surface color of a definite wave- length, the indices of refraction of the waves on the two sides of the absorbed waves are changed ; if their indices are less than that of the absorbed color for ordinary media, they are increased, and vice versa. This is known as 'anomalous dis- persion.' Absorptiox. When white light falls upon any material substance there is always a certain amount scattered from the surface, and also as a rule some scattered from the interior portions of the body if light enters. The light scattered from the surface is white, but if the body is a piece of polished metal or has a 'metallic lustre.' there is mixed with the white light some that is colored. This colored light is due to the fact that there is a certain amount of absorption of waves of a particular wave-length at the surface. Thus gold appears yellow, because out of tlic incident white light the bluish-green light enters at the surface, and the yellow light is re- fleeted. If a sheet of gold is hammered out ex- tremely thin, the transmitted light is bluish. If the light penetrates into a substance. and is either transmitted through or reflected nut by small solid particles serving as mirrors, the color is generally the same, being due to the absorption of certain waves in the body of the substance. (The energy of these absorbed waves goes generally into heat-effects, but may be spent in producing fluorescence, q.v. ) Some substances absorb two or more colors to different extents; so a thin plate of it will be of a different color from a thick one. As explained in the article on Color, the par- ticular color ordinarily attributed to a substance is that perceived by a normal eye when viewing the substance in white light. There are two methods for determining the cause of the color of an object. One method is to put it, if it is not opaque, between the slit and prism of a spectro- scope, the slit being illuminated by white light, and to notice the change in the spectrum. The resulting spectrum is called the 'absorption spec- trum' of the body, and the colors which are trans- mitted are the ones which, when combined in the eye, produce the color of the object. The other method is to throw a pure spectrum of Vol. XII.— 17. white light on a screen, and to move the object slowlj' along through the spectrum; if it apijears black when held in any color, it shows that the waves of this color are absorbed by the object. If two pieces of colored glass are superimp(jscd, or if two colored paints are mixed, the resulting color is that due to the waves which are left after each glass or each jiaint has subtracted its colors. Absorption must be due to the fact that the atoms' of matter can vibrate in definite periods; and if ether-waves of a proper period enter the substance they will by resonance set the portions of matter in vibration and so lo.se their own energy. If the energy thus gained by the atoms is dissipated among the molecules, increas- ing their energj', heat-effects are produced. It may happen, however, that atoms are set vibrat- ing in definite periods by the absorption of ether- waves of shorter period, and, instead of having their energy spent in hpat-eirecls, emit ether- waves of their own. This is called 'lluorescence' (q.v.). Thus a solution of quinine absorbs ether-waves whose period is sv suitable ther- mometers (q.v.), change in temperature is no- ticed. Waves longer than 0.007 cm. have not yet been observed as being emitted by ordinary mat- ter: waves longer than 0.2 em. may, however, be produced by electrical means. I'sing proper in- struments, it has been proved that solids and liquids emit continuous spectra; (hat is, waves of all wave-lengths between certain limits can be shown to be present. Gases, however, when rendered luminous by the passage of an electric discharge through them or when produced by vaporizing solids in the electric arc. give discon- tinuous spectra; that is, only isolated trains of waves are present. This is what would be ex- pected. In a gas the moleoules have great freedom of motion, and as one dashes to and fro having successive encounters with other molecules, its atoms have time during the intervals of free motion to emit long trains of waves of definite periods. But the molecules of solids and liquids are so close together and so hamper each other