The fact is, that not the objects them selves, but the picture formed upon the retina, is the immediate object of the sight. Without the sense of touch, it is probable that the picture would never have conveyed ideas of real figure, mag nitude, motion, or position ; still more, that it would never have conveyed the idea that external objects produced the picture. Of colour it does convey sensa tions, which do not receive correction from the touch, and which can be ac quired by the sight alone. Persons com pletely blind have been known to distin guish objects of one colour from those of another, but this is by the feel of the sur faces of those objects. If they have never at all possessed sight, though they may speakof colours, and distinguish coloured objects, and even have a remote idea of the causes of our sensations of colours, yet, they can have no sensations, nor consequently ideas of colours. Mr. Locke mentions a blind man, who said, that he imagined the colour of scarlet resembled the sound of a trumpet.
The limits here stated of the direct power of the sense of sight may appear strange to those who have not been ac customed to distinguish between the sen sation, and the perception of w:iich the sensation forms a part. (See SENSATION ) We seem to have an immediate sensation of the real situation and magnitudes, &c. of objects ; but what has been before stat ed is an indisputable fact. The case is, the compound idea, formed from the sen sations of touch, in connection with cer tain visual sensations, are so early form ed, and so early connected with those vi sual sensations, that we have no recollec tion of the simple idea of sensation, or of the formation of the compound idea ; in deed, as active agents, we had no concern in the formation of our perceptions. There are, however, numerous circum stances which prove the point ; the most satisfactory are those attending the obtaining of the sight, at a period when recollection can register the sensations. One such case fell under the observation of the able Cheselden, and we shall state some of the principal circumstances of it. Mr. C. couched a youth of thirteen years of age ; when he was allowed to use his sight, all objects appeared to him alike to touch his eyes, as the things which he felt touched his skin. He considered so lid bodies as planes differently coloured ; and when he had learned to distinguish solids by their appearances, lie was great ly surprised when examining the pictures of solids, to find all the parts plane and smooth like the rest ; he asked which of his senses deceived him, his sight or his hearing ? Being shown a miniature of his father, which was painted on a watch case, he at once perceived that it was a repre sentation of his father, but expressed great surprise that so large a counte nance could he contained in so small a space : it appeared to him as impossible as for a pint to hold a hogshead. Mr.
Ware published, in the Philosophical Transactions of 1800, a case which seemed to militate greatly against Mr. C.'s con clusions; Mr. W.'s patient from the first had ideas of distance and form. But Mr. W. himself furnishes a solution of this difficulty : for we find, from his paper, that his patient had always been able to distinguish light and vivid colours from shade.
Sensations of colour are in the early parts of life very vivid, and assist consi derably in the formation of our mental pleasures ; but the other sensations de rived from this sense are principally im portant to us, as being by association the signs of the ideas derived from the touch and from their distinctness well calculat ed to serve as the connecting bond of union, and to-bring again into the view of the mind those ideas. The visual sensa tions, of themselves considered, are sel dom the objects of reflection : we seldom even think of them, and while we appear to give to the visible appearances of ob jects our minutest attention, we are in part attending only to the tangible quali ties of which the visible appearance is the sign. Were it not therefore for as sociation, the sight would be of little more use to us than a beautiful picture of objects with which we have no concern. But consider its value in connection with association, and it must be regarded as the most perfect and the most perma nently valuable of all the senses. The information obtained by the touch is ac quired slowly ; and the sensations must be continually repeated, in order to ac quire information respecting new ob jects ; but the sight takes in a vast varie ty of objects, and, almost at a glance, can distinguish most that is necessary to be known respecting them. Its sensations recall the past impressions derived from the touch, and at once suggest the size, the shape, the distance of their various objects. " If a man," says Reid, "were by feeling to find out the figure of the Peak of Teneriffe, or even of St. Peter's at Rome, it would be the work of a life time " Besides, its discovery reaches farther than the touch could carry us ; it enables us to range through the vault of Heaven, and determine the motions of the heavenly luminaries. It traces in the countenance the workings of the mind it displays the passions and affections of the soul. With association it is every thing; without it, it would be useless as the bright fleeting visions of sleep.
Storrs of a quadrant, &c. thin pieces of brass, raised perpendicularly on its side, or on the index of a theodolite, circum ferentor, &c. They have each an aper ture, or slit, up the middle, through which the visual rays pass to the eye, and distant objects are seen.