es, three of which see everything
reversed, while the other two see things the right way up!

On the other hand, some regard each facet as an independent eye, in
which case many insects realise the epigram of Plato--

Thou lookest on the stars, my love,
Ah, would that I could be
Yon starry skies with thousand eyes,
That I might look on thee!

Even so, therefore, we only substitute one difficulty for another.

But this is not all. We have not only no proof that animals are confined
to our five senses, but there are strong reasons for believing that this
is not the case.

In the first place, many animals have organs which from their position,
structure, and rich supply of nerves, are evidently organs of sense; and
yet which do not appear to be adapted to any one of our five senses.

As already mentioned, the limits of hearing are reached when about
35,000 vibrations of the air strike on the drums of our ears. Light, as
was first conclusively demonstrated by our great countryman Young, is
the impression produced by vibration of the ether 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 luminous, though with light which is invisible to
us. In warmer climates the Fire-fly, Lanthorn-fly, and many other
insects, shine with much greater brilliance, and in these cases the
glow seems to be a real love-light, like the lamp of Hero.

Many small marine animals, Medusae, Crustacea, Worms, etc., are also
brilliantly luminous at night. Deep-sea animals are endowed also in many
cases with special luminous organs, to which I shall refer