THE PHYSICAL CAUSE OF ANIMAL COLOURS 3 



and it is certain that some animals can see vibrations 

 which do not affect our eyes. 1 The slowest vibrations 

 that we can see, produce the effect of red, the most 

 rapid the effect of violet, while the intermediate 

 vibrations cause the other well-known colours of the 

 rainbow or the spectrum. 



The absorption of certain elements of light there- 

 fore means the disappearance of ethereal vibrations 

 with a certain speed. It is believed that these 

 vibrations disappear because their motion has been 

 communicated to the particles of the absorbing body. 

 It is also believed that these particles are in a state of 

 constant vibration, and that the vibrations of ether, 

 which are timed to those of the body, are used up 

 in increasing the motion of the latter. 



A white appearance due to light being scattered 



The production of white is due to a different 

 principle, for we know that when light passes through 

 a body without any absorption, the body is trans- 

 parent and invisible rather than white. When all 

 the light passes through, the body is completely in- 

 visible. Whiteness is due to reflection of the whole 

 visible series of vibrations, unaccompanied by the 

 absorption of a part of them, as in the production of 

 colours. But regular reflection, viz. reflection from 



1 Sir John Lubbock, The Senses of Animals, Chapter X. (Inter- 

 national Scientific Series). 



