92 THE BEAUTIES OF NATURE CHAP. 
on the retina of the eye. When 700 millions 
of millions of vibrations strike the eye na 
second, we see violet; and the colour changes 
as the number diminishes, 400 millions of 
millions giving us the impression of red. 
Between 35 thousand and 400 millions of 
millions the interval is immense, and it is 
obvious that there might be any number of’ 
sensations. When we consider how greatly 
animals differ from us, alike in habits and 
structure, is it not possible, nay, more, is it 
not likely that some of these problematical 
organs are the seats of senses unknown to us, 
and give rise to sensations of which we have 
no conception ? 
In addition to the capacity for receiving 
and perceiving, some animals have the faculty 
of emitting light. In our country the glow- 
worm is the most familiar case, though some 
other insects and worms have, at any rate 
under certain conditions, the same power, and 
it is possible that many others are really lumi- 
nous, though with light which is invisible to 
us. In warmer climates the Fire-fly, Lan- 
thorn-fly, and many other ‘insects, shine with 
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