v COLOURS OF ANIMALS 355 
radiation, or, more commonly, rays; and consists of sets of 
waves which vary considerably in their dimensions and rate 
of recurrence, but of which the middle portion only is capable 
of exciting in us sensations of light and colour. Beginning 
with the largest waves, which recur at the longest intervals, 
we have first those which produce heat-sensations only ; as 
they get smaller and recur quicker, we perceive a dull red 
colour ; and as the waves increase in rapidity and diminish in 
size, we get successively sensations of orange, yellow, green, 
blue, indigo, and violet, all fading imperceptibly into each 
other. Then come more invisible rays, of shorter wave- 
length and quicker recurrence, which produce, solely or 
chiefly, chemical effects. The red rays, which first become 
visible, have been ascertained to recur at the rate of 458 
millions of millions times in a second, the length of each wave 
being ss3u0 Of an inch; while the violet rays, which last 
remain visible, recur 727 millions of millions times per second, 
and have a wavelength of gz#z, of an inch. Although the 
waves recur 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 recurrence, travel 
at the same speed, so that a tune played several hundred 
yards off reaches the ear in correct time. There are, there- 
fore, an almost infinite number of different colour-producing 
undulations, and these may be combined in an almost infinite 
variety of ways, so as to excite in us the sensation of all the 
varied colours 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 colour are prevented from reaching us, the remaining 
rays in combination produce a sensation of colour 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 comple- 
mentary colours. And if portions of differently coloured 
lights are abstracted in various degrees, we have produced all 
those infinite gradations of colours, and all those varied tints 
and hues which are of such use to us in distinguishing 
