CHAPTER IV. 



COLOUR, ITS NATURE AND RECOGNITION. 



THIS chapter will be devoted to a slight sketch of the nature 

 of light and colour, and to proofs that niceties of colour are 

 distinguished by animals. 



First, as to the nature of light and colour. Colour is essentially 

 the effect of different kinds of vibrations upon certain nerves. 

 Without such nerves, light can produce no luminous effect what- 

 ever ; and to a world of blind creatures, there would be neither 

 light nor colour, for as we have said, light and colour are not 

 material things, but are the peculiar results or effects of vibrations 

 of different size and velocity. 



These effects are due to the impact of minute undulations or 

 waves, which stream from luminous objects, the chief of which is 

 the sun. These waves are of extreme sraallness, the longest being 

 only 226 ten-millionths of an inch from crest to crest. The tiny 

 billows roll outwards and onwards from their scource at incon- 

 ceivable velocities, their mean speed being 185,000 miles in a 

 second. Could we see these light billows themselves and count 

 them as they rolled by, 450 billions (450,000,000,000,000) would 

 pass in a single second, and as the last ranged alongside us, the 

 first would be 185,000 miles away. We are not able, however, to 

 see the waves themselves, for the ocean whose vibrations they are. 

 is composed of matter infinitely more transparent than air, and 

 infinitely less dense. Light, then, be it clearly understood, is not 

 the ethereal billows or waves themselves, but only the effect they 

 produce on falling upon a peculiar kind of matter called the optic 

 nerve. When the same vibrations fall upon a photographic sensi- 

 tive film, another effect chemical action is produced : when they 



