88 THE BEAUTIES OF NATURE CHAP. 



violet are the limits of the visible spectrum, 

 they are not the limits of the spectrum itself, 

 there are rays, though invisible to us, beyond 

 the red at the one end, and beyond the violet 

 at the other : the existence of the ultra red 

 can be demonstrated by the thermometer ; 

 while the ultra violet are capable of taking 

 a photograph. But though the red and violet 

 are respectively the limits of our vision, I 

 have shown * by experiments which have been 

 repeated and confirmed by other naturalists, 

 that some of the lower animals are capable 

 of perceiving the ultra-violet rays, which to 

 us are invisible. It is an interesting question 

 whether these rays may not produce on them 

 the impression of a new colour, or colours, 

 differing from any of those known to us. 



So again with hearing, not only may 

 animals in some cases hear better than we 

 do, but sounds which are beyond the reach 

 of our ears, may be audible to theirs. Even 

 among ourselves the power of hearing shrill 

 sounds is greater in some persons than in 

 others. Sound, as we know, is produced by 



1 Ants, Sees, and Wasps, and The Senses of Animals. 



