92 THE BEAUTIES OF NATURE CHAP. 



on the retina of the eye. When 700 millions 

 of millions of vibrations strike the eye in a 

 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 



