VISION, the act of seeing; that faculty of the mind by reesras of which, through its appropriate material organ, the eye (q.v ), we are percipient of the visible appearances of the external world. Considered in the latter signification, vision includes questions of high importance in relation to some of the most iatricate„ nroblems of philosophy; as this part of the subject has already been discussed tinder the present article will be restricted, as far as possible, to an exposition of the phenomena and laws of vision proper. In opposition to the bulk of mankind, who believe undoubtingly that they actually see the externality and solidity of the bodies around them, bishop Berkeley maintained that these properties are not the immediate oojeets of sight at all, but are simply ideas derived originally from the touch, and erroneously attributed to vision, is consequence of their having been uniformly experienced concurrently with certain "visible signs" (as, for example, color), with which alone the sense of sight is truly con versant; and this theory of vision having since recei-red the adhesion of a great majority of the most able metaphysicians, it will be preper to give an outline of its leading pro positions. In doing this, we shall at the same dine intersperse such remarks and counter statements as may appear to be rendered necessary by the progress of opinion and the results of modern experimental inquiry First, as to the externality, or oldness of objects; or, which is the same thing, their distance from the eye. This, Berkeley main tains, cannot of itself and immediately be seen. "For distance being a line directed endwise to the eye, it projects only one point in the fund of the eye; which point remains invariably the same, whether the distance be longer or shorter." To this posi• Lion, everywhere assumed by Berkeley to be indisputable, and by his followers admitted to be so, it may be objected, that it contains an unwarranted assumption, viz., that a ray of light is, by its very nature, incompetent to convey an impression indicative of its possessing length or extension; or, to speak more accurately, it assumes that "apparent distance" is not at all affected by a variation in the actual length of the ray intervening between the eye and the object. Yet it seems obvious, that the facts of vision do not
admit of our arguing the matter, as though the line extending from any point of an object to the eye were a mere mathematical abstraction. Every visible point sends forth diverg ing rays, which form a cone whose base is on the pupil of the eye; and to the eye the place of this visible point is at the intersection, real or virtual, of the rays in question: real, when the radiant point is viewed directly; virtual, when the rays, either by refraction or reflection, are diverted from their original path before reaching the eye. To take a case of refraction: if we notice the distance of a shilling lying at the bottom of an empty vessel, we shall observe, upon filling the latter with water, a manifest diminution in the apparent distance of the shilling, the reason being that the rays, on their emergence from the water, are bent outward, so that the point of their virtual intersection is brought nearer to the eye. In reflection, the place of a visible point is, in like manner, referred to the point of virtual intersection of the cone of rays incident upon the pupil; and multiplied reflections, the apparent distance of a point actually adjacent to the eye, may he increased to an almost indefinite degree. It is forcibly contended by Berkeley that these facts, involving, as they do, geometrical considerations known only to few, and by none consciously realized in the act of vision, cannot be concerned in our appreciation of distance by the visive faculty. Yet these, and numberless similiar experiments, render it evident that both "apparent distance" and "apparent place" are closely depen dent upon these geometrical conditions; and, therefore, without assuming that vision is performed by the aid of connate or instinctive geometry (a notion justly condemned by Berkeley), it yet seems highly probable that these lines and angles are the exponents and invariable concomitants of an actual operation of light upon the eye, specific in its char acter, and by reason of its necessarily varying, pari 'avi, with every change in the dis tance of the point of intersection. of the visual rays, fitted to convey to us an intuitive perception of varying distance.