PDF - Wallace Online

356 TROPICAL NATUREexternal objects, and which form one of the great charms ofour existence. Primary colours would therefore be asnumerous as the different wave-lengths of the visible radiations,if we could appreciateall their differences; whilesecondary or compound colours, caused by the simultaneousaction 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 tous to be produced by combinations of three primary coloursred, green, and violet it is believed that we have threesets of nerve fibres in the retina, each of which is capable ofbeing excited by all rays, but that one set is excited most bythe larger or red waves, another by the medium or greenwaves, and the third set chiefly by the violet or smallestwaves of light ;and when all three sets are excited togetherin proper proportions we see white. This view is supportedby the phenomena of colour-blindness, which are explicable onthe theory that one of these sets of nerve-fibres (usually thatadapted to perceive red) has lost its sensibility, causing allcolours to appear as if the red rays were abstracted fromthem.It is a property of these various radiations that they areunequally refracted or bent in passing obliquely throughtransparent bodies, the longer waves being least refracted, theshorter most. Hence it becomes possible to analyse white orany other light into its component rays. A small ray ofsunlight, for example, which would produce a white spoton a wall, if passed through a prism, is lengthened out into aband of coloured light, exactly corresponding to the colours ofthe rainbow. Any one colour can thus be isolated andseparately examined ;and by means of reflecting mirrors theseparate colours can be again compounded in various ways,and the resulting colours observed. This band of colouredlight is called a spectrum, and the instrument by which thespectra of various kinds of light are examined is called aspectroscope. This branch of the subject has, however, nodirect bearing on the mode in which the colours of livingthings are produced, and it has only been alluded to in orderto complete our sketch of the nature of colour.The colours which we perceive in material substances are