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 ?6 ^ 60 - of an inch ; while the violet rays, which last 

 remain visible, recur 727 millions of millions times per second, 

 and have a wave-length of a ^ lfl 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 



