THE RAINBOW. 
253 
which we know to be harmonious, because divine; but 
the harmony is proved, by philosophy and experience, and 
the rainbow may be cited as the absolutely perfect ex¬ 
ample of the disposition of colours, for the gratification 
of human vision. 
But the prisms by which this spectrum is formed are 
not fixed, but moveable—rapidly moveable; no one of 
them for an instant keeps its place. How, then, is the 
immoveable, apparently solid, and eternal arch sustained 
in its prismatic completeness ? The question is easily 
answered. Just as in the glass prism the white light 
refranges, and each of its component parts becomes 
visible as a separate colour; so, for all practical pur¬ 
poses, the apparent fixity and permanence of the rainbow 
is secured by the means of myriads of falling prisms. If 
they did not fall, some other effect than that we witness 
would be the result. The rays of light from the sun 
are parallel, and for purposes of inquiry we may say 
there is but one ray, and out of that proceeds the 
variety of colours. So, instead of a million disjointed 
water drops, let us suppose there is but one, and on 
the surface of that one we desire the whole spectrum to 
be displayed. Now, in falling, the under side of that 
drop will first touch the rav: as it proceeds, the centre 
will come to the ray; and lastly, the ray will fall on the 
upper side of the drop, and at last above it altogether. 
At each of these several positions the angle of incidence 
will be such as to bring out the colours belonging to 
each ; and it is therefore evident, that if the colours 
were of the same degree of refrangibility, there could be 
no rainbow at all; and conversely, if their relative refran- 
