Nor is the difference one of degree merely. The rainbow, as we see it, consists of seven colours--red, orange, yellow, green, blue, indigo, and violet. But though the red and 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[16] 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 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.