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 eyes, 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?