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, Bees, and Wasps, and The Senses of Animals. 
- Ee 
