LIGHT AND COLOUR 



may trace out what happens to it in the follow- 

 ing way : The white light enters a very short dis- 

 tance into the leaf, but sooner or later gets turned 

 back by irregularities and obstacles within. In 

 its course it passes through a layer of matter 

 which takes up certain of the rays of white light 

 more than others, and keeps them behind in- 

 side, so that the light which finally comes out 

 and reaches the eye has lost some of its parts. 

 The balance is upset, and this light no longer 

 looks white ; it has the colour of those rays 

 which have not been taken away. 



When we go further, and ask how these 

 bodies take away and absorb some of the 

 coloured rays of white light, we are led into 

 much deeper questions questions over which 

 men have puzzled for many centuries. The 

 conclusion they have come to depends on evi- 

 dence that we may some day study as a branch 

 of physical science. We shall find that the 

 only satisfactory explanation is to regard light, 

 like sound, as a kind of wave motion. The 

 particles of coloured bodies can move, as it 

 were, in tune with certain kinds of waves more 

 freely than with others, and so the energy of 

 those waves is absorbed by the particles, and 

 the light passes on without them. 



If light consists of waves, there must be 

 some substance in which the waves can travel. 

 We have seen waves travelling in water; we 

 have found waves that travel in air. The sub- 



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