_Sight_ is the sense by which we take cognisance of those external objects which either give out or reflect light. The eye (fig. 84) is an optical instrument of great beauty and perfection, and by its help an image of everything we look at is produced in miniature on the expanded optic nerve called the "retina," as an image is produced in the "camera," which instrument is made in imitation of the eye; this image by some unknown power is perceived by the mind, and produces the sense of vision. By the sense of sight we recognise all forms, colours, and gradations of light and shade, and also (assisted by the experience gained by the sense of touch) the qualities of surface, such as roughness, smoothness, &c.; for instance, we know by touch that a piece of glass is smooth, that a piece of velvet has a different kind of smoothness, &c., and the mode in which these surfaces reflect the light, teaches us to recognise them so that we could predict from touch what appearance an object would have, or by the look how it would feel. Our knowledge of the solidity of bodies is known to the sense of sight by (unconsciously) looking at them with each eye alternately, so as to see to a certain extent round them, or somewhat on either side alternately. By this fact the principle of the stereoscope is explained, the pictures looked at through it, consist of two views of the same object, taken at points of sight a short distance apart (corresponding to the distance between the eyes) and they give us, when steadily looked at, the same result that real objects do when looked at with each eye, alternately. It is a curious fact, but one which has been clearly demonstrated by optical experiments, that the picture represented on the "retina" is inverted, but from habit we refer things to their natural position, and how naturally and easily this is done, may be ascertained by holding the head down close to the floor, and looking at objects through one's legs, in this position they are all inverted; but we naturally make ourselves satisfied that they are still as they were before, so that they really do not appear to be inverted. The impression formed by objects upon the retina is not immediately destroyed upon their removal, but remains for an instant permanent; this gives rise to a great many curious phenomena, among which the most familiar is the circle of light produced by the rapid evolution of any luminous body, as the red-hot end of a stick. This is also shown by a toy, called the "Thaumatrope," which consists of a piece of card having a small wire attached to the upper part by which it may be turned rapidly round by the thumb and finger. On one side of the card is painted an object, such as a parrot, and on the other a cage; when the card is turned round, first the bird and then the cage is presented to the eye, but the image of the one has not left the eye before the other is presented to it, and the result is the uniform appearance on