SIGHT, sense of. The organ of sight is a globular body, which contains within it transparent substances, fitted to form on the back part of it a picture of the ob ject of sight. An examination of the eye of an ox will give a pretty accurate idea of the general structure of the human eye. We see in front a horn) transpa rent substance, called the cornea. Next to this is a watery fluid, called the aque ous humour, in which the iris floats like a delicate curtain, with a hole in the mid dle, called the pupil. Behind the iris we find a solid body with two convex sur faces ; this is called the crystalline lens. Next to this is the vitreous humour, a jelly like, transparent substance, which fills the ball of the eye. The retina con sists of exceedingly minute fibres from the optic nerve, which are spread over the whole of the back part of the inner surface of the eye. Behind the retina is a mucous or slimy matter, which in the human eye is of a dark colour, and serves to imbibe the rays of light which pass through the retina, so as to prevent the confusion which would arise from the re flection of them. The retina is the imme. diate organ of sight. The rays of light, proceeding from every visible point of the object of sight, enter the eye through the cornea, and pass through the pupil; they are refracted by the three humours of the eye, so as to form upon the retina an exquisitely beautiful and distinct, though minute, picture of the object. A pretty correct idea of the formation of this picture may be obtained, by care fully cutting oft' the back of an ox's eye, and holding behind it a piece of Paper to receive the picture of a lumi nous object. For a more minute occount of the structure of the eye, see AL. TOMS'.
What effect is produced upon the op tic nerve by the formation of this picture upon the retina is not certainly known; it is sufficient for our present purpose, that, by means of the nerve, &c. the Impression, whatever it be, is communicated to the mental organs, and produces in them those effects, which, when attended with consciousness, are called sensations. See MSISTAL PHILOSOPHY, § 11.
If the sensation produced by the object of sight be considered, unblended with the relics of other sensations, we find that it is merely what can be communicated by a minute picture on the retina. The
sensation of colour can be thus communi cated, and this is the only sensation which can be considered as appropriate to the sight. The sensation of figure can be thus communicated, but only of figure in two directions, length and breadth : for the picture on the retina can have only those two dimensions. The sensation of magnitude.ean also be thus communicat ed, but not of real magnitude ; for the vi sible sensation of real magnitude cannot be conveyed by a picture, which is al most indefinitely smaller than the real object. To use the illustration of Adam Smith : " If you shut one eye, and hold immediately before the other a small cir cle of plain glass, of not more than half an inch in diameter, you may see through that circle the most extensive prospects, lawns, and woods, and arms of the sea, and distant mountains. You are apt to ima gine that the visible picture, which you thus see, is immensely great and exten sive; but it can be no greater than the visible circle through which you see it. If, while you are looking through the cir cle, you could conceive a fairy hand and a fairy pencil to come between your eye and the glass, that pencil might delineate upon that little glass the outline of all those extensive lawns, and woods, and arms of the sea, and distant mountains, in the dimensions with which they are seen by the eye." Again, it is obvious, that however large, or however small, the field of view, the picture occupies an equal extent on the retina. Similar ob servations may be made with respect to distance. The organ of sight can con vey only that sensation of distance which may be produced by a minute picture on the retina; that is, nothing but the sensa tion of the distances of the different parts of the picture, which may bear no pro portion to the real distances, and can on ly be in one direction. Similar things may bg said of motion, that is, change of position. The visible sensation of motion is merely that produced by the motion of different parts of the picture on the retina.