SIGHT (vision, the faculty of seeing). The structure and uses of the several component parts which enter into the formation of the organ of vision have been already described in the article EYE, in NAT. Hisr. Div. We have now to inquire by what means the images of objects which are depicted on the retina become converted into ideas of the objects themselves; of their proximity and distance ; of their solidity and size. In other words, is the interpretation of the sensa tional of the retina a vital property of the structure itself, or is it in part derived from other sources I The following case, which is recorded by Cheselden, affords us important data on this head. A young man, who was born blind, was suddenly restored to sight by the operation of couching. When he first saw," observes Cheselden, "he was so far from making any judgment about distances, that he thought all objects whatever touched his eye (as he expressed it), as what he felt touched his skin. He knew not one thing from another, however different in shape and magnitude ; but upon being told what things were whose form ho before knew from feeling, ho would carefully observe, that he might know them again. Two mouths after being couched, his attention seems to have been drawn to the effects of painting, which he then first and at once comprehended; but even then be was no less surprised, expecting [the pictures would feel like the things they re presented, and was amazed when he found those parts which by their light and shadow appeared round and uneven, felt only flat like the rest, and asked which was the lying sense, feeling or seeing / Being shown a small miniature of his father, and told what it was, he ac knowledged a likeness, but was vastly surprised, asking how it could be that a large face could be expressed in so little room, saying it should have seemed as impossible to him as to put a bushel into a pint. At first he could bear but very little light, and the things ho saw he thought extremely large, but upon seeing things larger, those first seen he conceived lose, never being able to imagine any lines beyond the bounds he saw. The room he was in, be said he blew to be but part of the house, yet he could not conceive that the whole house could look bigger." From the details of this interesting case, it would appear that the sense of eight (so far as we can judge of it when per formed with one eye, for only one had been operated on when the above phenomena were observed) originally gives ue no information respecting the solidity, the distance, or the real magnitude of objects ; but that they all seem as if pointed on one surface. If this then is the sum of the information which is conveyed to us by the retina ; if it is limited to the mere perceptions of the images of objects, and conveys to us ideas relative to superficial extent only, it is clear that our estimation of the true position, the magnitude, and solid forms of bodies must be duo to soihe other sense than that of sight, or rather, to a comparison of some other sense with it ; in short, to an act of our comparing and reasoning faculties.
We have seen by the details of the above quoted case, that there is no essential resemblance between the ideas which are derived from vision and those communicated by touch ; and it is no doubt partly owing to this circumstance that we obtain a correct knowledge of external objects through our visual organs. The lad couched by Cliesehlen could not recognise by sight the things whose form lie before knew_ from feeling ; but upon being told what they were, he would carefully observe, that he might know them again. The infant, in like manner, stretches out its little hands to grasp and examine each object in succession which attracts its sight, and the greater part of its waking hours is employed in thus comparing the sensations obtained through these two different channels. That we do acquire important information respecting the size and forma of bodies through the sense of touch there can be no doubt ; that the knowledge obtained through our visual organs would be Imperfect without it, and that it may in some measure be a corrective of those optical illusions which are so frequent when we attempt to judge of things by sight alone, is equally probable; but in admitting this, we must not underrate the original powers of the eye and the quantity of knowledge which we primarily derive from it. From some facts we are about to notice, it would appear that much of the information which we derive from our visual organs only, has hitherto been attributed to extraneous sources. Phy siologists have been too much swayed by the opinion of Gassendus, Haller, Gall, and othero, that we see with only one eye at a time, and those even who disputed this have been more anxious to explain why objects are seen single with both eyes than to inquire into the uses of our possessing two. These defects, which are more or less common to all writers on optics, have been some years since remedied by some very interesting observations of Professor Wheatstone on Binocular Vision. He has shown that the simultaneous affection of the two retinae, provided the optic axes are not parallel, excites a different idea in the mind from that couscquent on either of the single impressions; the latter giving rise to a representation on a plane surface, the fonner to that of an object in relief. This is owing to a different perspective projection of the object being seen by each eye; thus, if any figure of three dimensions, an outline cube for example, is held at the distance of about seven inches before the eyes, and viewed with each eye suc cessively while the head is kept perfectly steady, a will be the picture presented to the right eye, and B that seen by the left. Now if these two pictures are made to fall on corresponding parts of the retina;, by placing them one in the direction of each optic axis at equal distances before or behind their intersection, the mind will perceive not merely a single representation of the object, but a body projecting in relief, the exact counterpart of that from which the drawings were made.