VISION, is the act of seeing or of per ceiving external objects by the organ of sight. As every point of an object, A B C, (Plate XVI. Miscel. fig. 11.), sends out rays in all directions, some rays from every point on the side next the eye, will fall upon the cornea, between E and F, and by passing on through the humours and pupil of the eye, they will be con verged to as many points on the retina, or bottom of the eye, and will thereon form a distinct inverted picture, c b a, of the object. Thus the pencil of rays, g r s, that flows from the 'point, A, of the oh ject, will be converged to the point, a, on the retina ; those from the point, B, Will be converged to the point b ; those from the point, C, will be converged to the point c ; and so of all the interme diate points, by which means the whole image, a b c, is formed, and the object made visible, although it must be owned that the method by which the sensation is carried from the eye by the optic nerve to the common sensory m the brain, and discerned, is above the reach of our conception. That vision is effected in this manner may be demonstrated expe rimentally. Take a bullock's eye, while it is fresh, and having cut off the three coats from the back part, quite to the vitreous humour, put a piece of white paper over that part, and hold the eye to wards any bright object, and you will see an inverted picture of the object upon the paper. The diameters of images at the bottom of the eye are proportional to the angles which the objects subtend at the eye, the same as in a lens, and are reciprocally as the distances of the same object vietved in different places. The eye is in reality no more than a camera obscura; for the rays of light flowing tom all the points of an object, through. Wle pupil of the eye, do, by the refraction °fits humours, paint the image thereof in the bottom of the eye ; just so it is lit the camera obscura, where all the rays refracted by a lens in the -window-shut ter, or passing through a small hole in it, paint the image on the opposite wall. Some properties of the eye are these : the eye can only see a very small part of an object distinctly at once. For the col lateral parts of an object are not repre sented distinctly in the eye ; and there fore the eye is forced to turn itself suc cessively to the several parts of the ob ject it wants to view, that they may fall neat- the axis of the eye, where alone distinct vision is performed. When any point of an object is seen distinctly with both eyes, the axis of both eyes are di rected to that point, and meet there ; and then the object appears single, though looked at with, both eyes ; for the optic nerves are so framed, that the cor respondent parts in both eyes lead to the same place in the brain, and give but one sensation, and the image will be twice as briiht with both eyes as with one. But if
the axis of both eyes be not directed to the object, that object will appear dou ble, as the pictures in the two eyes do not fall upon correspondent or similar parts of the retina. The best eye can hardly distinguish any object that sub tends at the eye an angle less than half a minute, and very few can distinguish it when it subtends a minute. If the dis tance of two stars in the heavens be not greater than this, they will appear as one. Though men, may see distinctly at differ ent distances, by altering the position and figure of the crystalline, yet they can only see distinctly within certain limits, and nearer than that, objects appear con fused. But these limits are not the same in different people. A good eye can see distinctly when the rays fall parallel up on it, and then the principal focus is at the bottom of the eye ; a man can judge at a small distance, with a single eye, by Trequenify observing how much variation is made in the eye to make the object distinct, and from this a habit of judging is acquired. But this cannot be clone at great distances, because, though the distance be varied, the change iii the eye becomes then insensible. But a man can judge of greater distances with both eyes, than he can with one. For the eyes being at a distance from one another, as long as that distance has a sensible pro portion to the distance of the object, one gets a habit of judging, by the position of the axis of the eyes, which are always directed to that point. For different dis tances require different positions of the axis, which depend on the motions of the eyes, which we feel. But in very great dista,nces no judgment ca,t lie, made from the motion of the eyes, or their internal parts. Therefore we can only guess at the distaff from the magnitudg, co and position n of interjacent bo dies. Dimness of sight generally attends old people, and this may arise from two causes. 1. By the eyes growing flat, and not uniting the rays at the retina,. which causes indistinctness of vision ; or, 2. By the opacity of the humours of the eye, which in time lose their transparency, in some degree ; from whence it follows; that a great deal of the light thk enters the eye is stopped and loot, and every object appears faint and dim. Hence the necessity of spectacles.