656 The Colors of Animals and Plants. —_[ November, 
and the third set chiefly by the violet or small waves of light ; 
and when all three sets are excited together in proper propor- 
tions we see white. This view is supported 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 various 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 possi- 
ble to analyze white or any other light into its component 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 corresponding 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 col- 
ors observed. This band of colored light is called a spectrum, 
and the instrument 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 substances are pro- 
duced either by the absorption or by the interference of gote of 
the rays which form white light. Pigmental or absorption col- 
ors are the most frequent, comprising all the opaque tints of flow- 
ers and insects, and all the colors of dyes and pigments. They 
are caused by rays of certain wave-lengths being absorbed, ya! 
the remaining rays are reflected and give rise to the sensation © 
color. When all the color-producing rays are reflected in e 
proportion the color of the object is white ; when all are absorbet 
the color is black. If blue rays only are absorbed the resulting 
color is orange-red ; and generally, whatever color an object Ka 
pears to us, it is because the complementary colors are absor vi 
by it. The reason why rays of only certain refrangibilities od 
reflected, and the rest of the incident light absorbed by esi 
stance, is supposed to depend upon the molecular structure iss 
body. Chemical action almost always implies change of mo oa 
lar structure ; hence chemical action is the most potent icine i 
$ x , gia i effects 4 
change of color. Sometimes simple solution in water 67%" 
marvelous change, as in the case of the well-known aniline dyes; — 
