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 vibration of the air striking on the drum of the ear, and
the fewer are the vibrations in a second, the deeper is the sound, which
becomes shriller and shriller as the waves of sound become more rapid.
In human ears the limits of hearing are reached when about 35,000
vibrations strike the drum of the ear in a second.

Whatever the explanation of the gift of hearing in ourselves may be,
different plans seem to be adopted in the case of other animals. In many
Crustacea and Insects there are flattened hairs each connected with a
nerve fibre, and so constituted as to vibrate in response to particular
notes. In others the ear cavity contains certain minute solid bodies,
known as otoliths, which in the same way play upon the nerve fibres.
Sometimes these are secreted by the walls of the cavity itself, but
certain Crustacea have acquired the remarkable habit of selecting after
each moult suitable particles of sand, which they pick up with their
pincers and insert into their ears.

Many insects, besides the two large "compound" eyes one on each side of
the head, have between them three small ones, known as the "ocelli,"
arranged in a triangle. The structure of these two sets of eyes is quite
different. The ocelli appear to see as our eyes do. The lens throws an
inverted image on the back of the eye, so that with these eyes they must
see everything reversed, as we ourselves really do, though long practice
enables us to correct the impression. On the other hand, the compound
eyes consist of a number of facets, in some species as many as 20,000 in
each eye, and the prevailing impression among entomologists now is that
each facet receives the impression of one pencil of rays, that in fact
the image formed in a compound eye is a sort of mosaic. In that case,
vision by means of these eyes must be direct; and it is indeed difficult
to understand how an insect can obtain a correct impression when it
looks at the world with five ey