Page:The American Cyclopædia (1879) Volume V.djvu/101

COLOR 97 ciprocate mainly the blue and allied rays, it will be some shade of blue ; and so of all colors. Moreover, whatever rays the molecular condi- tion of the body forbids it to reciprocate, it will of necessity extinguish ; but as force can no more be destroyed than matter, this extinguish- ed light is not lost, but becomes manifest again as heat in the body receiving the rays, or per- haps in certain circumstances as affinity or electricity. According to Brewster's theory, a body is exactly of the color which it rejects, or does not possess ; according to Euler's, the body is of the color it shows, and color is not a quality of light merely, but a secondary phys- ical quality of bodies, due to a primary me- chanical one. Thus a white body, as a screen, can respond to any color thrown upon it ; but a colored body can only respond to rays within a certain limit, and if it receive rays only of colors beyond this limit, it must appear black. In the undulatory theory, as now received by the highest authorities in physics, there is as- sumed to be a fixed and invariable connection between the color of a given ray and its re- frangibility ; and the latter again depends on its rate and time of vibration. Thus, as cal- culated by Dr. Young, the middle red ray has a wave length of ^-.J-g-o f an inch, and cor- responds to 477" millions of millions of pulsa- tions per second of the ether, or of the retina; while the middle violet has a wave length of BT.TT^ f an inch, and corresponds to 699 millions of millions of pulsations per second ; the other colors having wave lengths interme- diate to these. Sir David Brewster, however, who set out with a partiality for the now ex- ploded corpuscular theory, examined the spec- trum with variously colored glasses, and de- clared that he detected some red, yellow, and blue in all parts of it. He accordingly pro- posed a theory of white light as composed of three elementary colors only, those just named ; each color having all the various degrees of refrangibility, and the other colors being mix- tures of these three. Helmholtz has shown that many of Brewster's results were due to using imperfect prisms, and that when these sources of error are in a high degree removed, the mixture of rays through the spectrum fails to appear. Complementary Colors. Solar or white light being regarded as a whole pro- duced by the union of many elements (colors), it is plain that any portion of these elements being blended to form a composite hue, the remainder will form by mixture some other hue; and either of these being added to the other, the result will be the reproducing of complete or white light. In mathematical lan- guage, that which by addition to any part completes it, or makes a whole, is termed a complement. Transferring the mathematical conception to light, any hue, simple or com- pound, so related to another that by blending it produces with the latter white light, is term- ed its complementary color, and vice versa. Suppose the four higher prismatic colors blend- ed ; their dominant hue is blue. If now the three lower colors, red, orange, and yellow, be mingled in a separate hue, this will be an orange- red. But this combined with the compound blue just named must produce complete or white light. Hence, in the particular hues of each thus obtained, blue and orange-red are complementary ; but there may be many slight variations of these hues which shall go by the same names, and yet not be exact comple- ments, the result of their mixture being then a white tinged with whatever color happens to be in excess ; i. e., a light tint of that color. All the colors but yellow and green being blended give a hue of violet-red ; and of this the yellow-green is complementary. If one of the compound hues be varied by the incor- poration of more of a certain element, the other must lose an equivalent of the same ele- ment. If the yellow-green be moved toward green by adding blue, the violet-red, in order still to be complementary, must be moved toward red, by losing the same amount of blue as the other gains. If one of the hues be, ir- respective of light or shade, increased in inten- sity, the other must also be made more intense ; since, if the relative quantities of each color existing in the normal spectrum be not main- tained, an uncompensated portion of one or more colors must remain. The following pairs of colors are complementary to each other: Red bluish-green ; orange blue (azure) ; yel- low indigo ; green reddish-violet ; blue orange-red ; indigo orange-yellow ; violet yellow-green. M. Ohevreul, superintendent of the dyeing department of the Gobelins man- ufactory, constructed a chart of 24 hues, each shown in 24 different tones, and so arranged that not only the composition and relations, but also the complementary color of each of these, are obvious at a glance. Subjective or Acciden- tal Colors. Place on a sheet of white paper a red wafer, and look on it intently for some seconds ; move the wafer suddenly away, and in its place is seen a bluish-green spot of sen- sibly the same size. After looking at an orange wafer an azure image will appear ; indigo will follow yellow, and violet succeed yellowish green, and the reverse. The color of the image is in all cases complementary to that of the original object. If small objects of any colors be well illuminated and viewed on a black ground, the complementary image is seen upon closing the eyes. Under the same circumstances, a white object will give a black image. The color thus following upon the contemplation of its complement has been termed accidental, and also, from the fact that it arises from some state produced within the visual organ itself, subjective. A complete account of the ordinary phenomenon may be summed up thus : The color of the object itself grows gradually more faint, as it is viewed; after the cessation of the direct action of the object on the retina, there is, 1st, the persis- tence for about -fa of a second of the primitive