COLOURS OF ANIMALS. 181 



travel at the same speed, so that a tune played several 

 hundred yards off reaches the ear in correct time. There 

 are, therefore, an almost infinite number of different 

 colour-producing undulations, and these may be com- 

 bined 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 sensa- 

 tion 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 complementary colours. 

 And if portions of differently coloured lights are ab- 

 stracted 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 

 external objects, and which form one of the great 

 charms of our existence. Primary colours would there- 

 fore be as numerous as the different wave-lengths of the 

 visible radiations, if we could appreciate all their differ- 

 ences ; while secondary or compound colours, 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 colours 

 appear to us to be produced by combinations of three 

 primary colours 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, 



