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THE POPULAR SCIENCE MONTHLY.— SUPPLEMENT. 



color are prevented from reaching us, the re- 

 maining rays in combination produce a sensation 

 of color often very far removed from white. Thus 

 green rays being abstracted leave purple light ; 

 blue, orange-red light ; violet, yellowish-green 

 light, and so on. These pairs are termed com- 

 plementary colors. And if portions of differently 

 colored lights are abstracted in various degrees, 

 we have produced all those infinite gradations of 

 colors, and all those varied tints and hues, which 

 are of such use to us in distinguishing external 

 objects, and which form one of the great charms 

 of our existence. Primary colors would there- 

 fore be as numerous as the different wave-lengths 

 of the visible radiations if we could appreciate 

 all their differences, while secondary or com- 

 pound colors caused by the simultaneous action 

 of any combination of rays of different wave- 

 lengths must be still more numerous. In order 

 to account for the fact that all colors appear to 

 us capable of being produced by combinations of 

 three primary colors— red, green, and violet — it 

 is believed that we have three sets of nerve-fibres 

 in the retina, each of which is capable of being 

 excited by all rays, but that one set is excited 

 most by the larger or red waves, another by the 

 medium or green waves, and the third set chiefly 

 by the violet or small waves of light ; and 

 when all three sets are excited together in proper 

 proportions we see white. This view is sup- 

 ported by the phenomena of color-blindness, 

 which are explicable on the theory that one of 

 these sets of nerve-fibres (usually that adapted to 

 perceive red) has lost its sensibility, causing all 

 colors to appear as if the red rays were abstracted 

 from them. It is another property of these va- 

 rious radiations that they are unequally refracted 

 or bent in passing obliquely through transparent 

 bodies, the longer waves being least refracted, 

 the shorter most. Hence it becomes possible to 

 analyze white or any other light into its compo- 

 nent rays ; a small ray of sunlight, for example, 

 which would produce a round white spot on a 

 wall, if passed through a prism is lengthened out 

 into a band of colored light exactly correspond- 

 ing to the colors of the rainbow. Any one color 

 can thus be isolated and separately examined, 

 and by means of reflecting mirrors the separate 

 colors can be again compounded in various ways, 

 and the resulting colors observed. This band of 

 colored light is called a spectrum, and the instru- 

 ment by which the spectra of various kinds of 

 light are examined is called a spectroscope. This 

 branch of the subject has, however, no direct 

 bearing on the mode in which the colors of living 



things are produced, and it has only been alluded 

 to in order to complete our sketch of the nature 

 of color. 



The colors which we perceive in material sub- 

 stances are produced either by the absorption or 

 by the interference of some of the rays which 

 form white light. Pigmental or absorption colors 

 are the most frequent, comprising all the opaque 

 tints of flowers and insects, and all the colors of 

 dyes and pigments. They are caused by rays of 

 certain wave-lengths being absorbed, while the 

 remaining rays are reflected and give rise to the 

 sensation of color. When all the color-producing 

 rays are reflected in due proportion the color of 

 the object is white, when all are absorbed the 

 color is black. If blue rays only are absorbed 

 the resulting color is orange-red ; and generally, 

 whatever color an object appears to us, it is be- 

 cause the complementary colors are absorbed by 

 it. The reason why rays of only certain refran- 

 gibilities are reflected and the rest of the incident 

 light absorbed by each, substance, is supposed to 

 depend upon the molecular structure of the body. 

 Chemical action almost always implies change of 

 molecular structure, hence chemical action is the 

 most potent cause of change of color. Some- 

 times simple solution in water effects a marvel- 

 ous change, as in the case of the well-known 

 aniline dyes ; the magenta and violet dyes exhib- 

 iting, when in the solid form, various shades of 

 golden or bronzy metallic green. Heat, again, 

 often produces change of color, and this without 

 effecting any chemical change. Mr. Ackroyd has 

 recently investigated this subject, 1 and has shown 

 that a large number of bodies are changed by 

 heat, returning to their normal color when cooled, 

 and that this change is almost always in the di- 

 rection of the less refrangible rays or longer wave- 

 lengths ; and he connects the change with molec- 

 ular expansion caused by heat. As examples may 

 be mentioned mercuric oxide, which is orange- 

 yellow, but which changes to orange, red, and 

 brown, when heated ; chromic oxide, which is 

 green, and changes to yellow ; cinnabar, which 

 is scarlet, and changes to puce ; and metaborate 

 of copper, which is blue, and changes to green 

 and greenish-yellow. The coloring-matters of 

 animals are very varied. Copper has been found 

 in the red of the wing of the turaco, and Mr. 

 Sorby has detected no less than seven distinct 

 coloring-matters in birds'-eggs, several of which 

 are chemically related to those of blood and bile. 

 The same colors are often produced by quite dif- 



1 " Metachromatism, or Color-ChaDgc " (Chemical 

 Neics, August, 1876). 



