1877.] | The Colors of Animals and Plants. 655 
blue, indigo, and violet, all fading imperceptibly into each other. 
Then come more invisible rays, of shorter wave-length and 
quicker vibration, which produce, solely or chiefly, chemical ef- 
fects. The red rays, which first become visible, have been as- 
certained to vibrate at the rate of four hundred and fifty-eight 
millions of millions of times in a second, the length of each wave 
being sg}yy Of an inch; while the violet rays, which last re- 
main visible, vibrate seven hundred and twenty-seven mill- 
ions of millions of times per second, and have a wave-length 
of gz}y¢ Of an inch. Although the waves vibrate at different 
rates, they are all propagated through the ether with the same 
velocity (192,000 miles per second), just as different musical 
sounds, which are produced by waves of air of different lengths 
and rates of vibration, travel at the same rate, so that a tune 
played several hundred yards off reaches the ear in correct time. 
There are, therefore, an almost infinite number of different color- 
producing vibrations, and these may be combined in an almost 
infinite variety of ways, so as to excite in us the sensation of all 
the varied colors and tints we are capable of perceiving. When 
all the different kinds of rays reach us in the proportion in which 
they exist in the light of the sun, they produce the sensation of 
white. If the rays which excite the sensation of any one color 
are prevented from reaching us, the remaining rays in combina- 
tion 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 complementary 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 dis- 
Unguishing external objects, and which form one of the great 
charms of our existence. Primary colors would therefore be as 
humerous as the different wave-lengths of the visible radiations, 
if we could appreciate all their differences ; while secondary or 
compound colors, caused by the simultaneous action of any com- 
bination of rays of different wave-lengths, must be still more nu- 
merous. In order to account for the fact that all colors appear 
to us capable of being produced by combinations of three primary 
lors, — red, green, and violet, — it is believed that we have 
ae 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, 
