356 TROPICAL NATURE v 
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 
