356 TROPICAL NATURE 



external objects, and which form one of the great charms of 

 our existence. Primary colours would therefore be as 

 numerous as the different wave-lengths of the visible radia- 

 tions, if we could appreciate all their differences; 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, but that one set is excited most by 

 the larger or red waves, another by the medium or green 

 waves, and the third set chiefly by the violet or smallest 

 waves of light ; and when all three sets are excited together 

 in proper proportions we see white. This view is supported 

 by the phenomena of colour-blindness, which are explicable on 

 the theory that one of these sets of nerve-fibres (usually that 

 adapted to perceive red) has lost its sensibility, causing all 

 colours to appear as if the red rays were abstracted from 

 them. 



It is a property of these various radiations that they are 

 unequally refracted or bent in passing obliquely through 

 transparent bodies, the longer waves being least refracted, the 

 shorter most. Hence it becomes possible to analyse white or 

 any other light into its component rays. A small ray of 

 sunlight, for example, which would produce a white spot 

 on a wall, if passed through a prism, is lengthened out into a 

 band of coloured light, exactly corresponding to the colours of 

 the rainbow. Any one colour can thus be isolated and 

 separately examined ; and by means of reflecting mirrors the 

 separate colours can be again compounded in various ways, 

 and the resulting colours observed. This band of coloured 

 light is called a spectrum, and the instrument by which the 

 spectra of various kinds of light are examined is called a 

 spectroscope. This branch of the subject has, however, no 

 direct bearing on the mode in which the colours of living 

 things are produced, and it has only been alluded to in order 

 to complete our sketch of the nature of colour. 



The colours which we perceive in material substances are 



