182 TROPICAL NATURE, AND OTHER ESSAYS. 



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 round 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 ex- 

 amined ; 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 produced either by the absorption or by the inter- 



