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be agreeable to the eye. The inner coat is a fine expansion of the optic nerve, which passing through the other coats, spreads like net-work over the inside of the choroides, and is therefore denominated the retina. According to some philosophers, the images of external objects are painted upon this coat, and by the impressions of light upon it, the ideas are communicated to the brain, the common origin of all the nerves. Although others are of opinion, that the images are painted on the choroides, and that the sensations are communicated to the brain by means of its own nerves. The principal arguments for the support of these opinions we shall see presently.

The three humours of the eye are the aqueous, the crystalline, and the vitreous. The aqueous humour lies immediately behind the cornea, and makes the eye globular before. It is divided into two portions, a part lying before, and a part behind the iris, and communicating with each other by means of the pupil. It derives its name from its resembling water in its limpidity, consistence, specific gravity, and refractive power. As the rays of light enter this denser medium through the convex surface of the cornea, they are refracted toward the axis of the eye, and pass through the pupil.

Behind the aqueous humour lies the crystalline, which in shape resembles a double convex lens, being more convex on the back part than before. Being of a lenticular form, it serves to give a greater degree of convergency to the rays, that are refracted through the aqueous humour. It is transparent as crystal, of the consistence of a hard jelly, and exceeds the specific gravity of the aqueous humour in the ratio of 11 to 10. It is inclosed in a fine transparent membrane call

ed its capsula and floats in a thin pellucid fluid like water, without any connexion with the capsula; but from the edge of the capsula, all round, proceed radial fibres, denominated the ligamenta ciliaria, which connect it with the choroides at the beginning of the iris and cornea, so that it lies between the two segments of the eye. By the ligamenta ciliaria contracting or dilating, the form of the crystalline lens, or at least of the fluid in which it floats, may be altered so as to make it less or more convex; by which means, it may refract the rays that proceed from objects of different distances to the same focus, and thereby afford a distinct vision of objects at different distances.

Behind the crystalline lens, lies the vitreous humour, which is also transparent like glass, and nearly of the same refractive power. It is nearly of the consistence of the white of an egg, exceeding the aqueous humour a little in its specific gravity and refractive power, filling the larger segment of the eye, and giving it a globular figure. As it is of less density than the crystalline lens, the rays which enter it through its concave surface are made to converge more towards the axis of the eye; so that all the humours of the eye contribute something to the converging of the rays of light.

The rays that issue from the object falling upon the eye are refracted by its humours, so as to fall upon the retina, where they form an inverted image of the object, by means of which we are made sensible of its existence, position, and motions, although in a manner to us incomprehensible. Yet that vision is performed by means of these images of external objects painted in the eye, is extremely probable from the very existence of them, of which any person may assure himself, by taking away the coats from the back

parts of the eye, and presenting it to a candle, whose image will then be painted and inverted on a piece of white paper placed on the back part of the vitreous humour.

The different humours of the eye are of such a determinate degree of convexity and refractive power, as to converge all the rays of light belonging to any pencil accurately into the same focal point on the retina, notwithstanding their different refrangibility; so that external objects may appear without those colours, which arise from the rays of different sorts in the same pencil being refracted to different points, where they form an image of the colour peculiar to each species of them. The different forms and densities of the humours of the eye lead us to believe that the design of Providence, in the construction of it, was to remedy the inconvenience arising from the different refrangibility of the rays of light. The aqueous and vitreous humours have the form of a meniscus, and the crystalline is a double convex lens, and by their composition, being of different densities, the most refrangible as well as the least refrangible rays of any pencil are accurately collected into a point, where the beam must appear white; so that external objects will appear in their own natural colours, and without any thing of that colouring, which arises from the dispersion of the rays of the same pencil.

Vision might be performed without the crystalline lens, but obscurely, since without it the rays would be brought to a focus beyond the retina; as is evident from the case of persons, who have suffered the extraction of it, on account of its having become opake.

As the image of the object is inverted in the eye, many have wondered how the object should appear to

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us to be erect. But we should remember that erect and inverted are only relative terms. And when all the parts of a distant prospect are painted on the retina, they all preserve their relative positions with respect to each other; and we have no means of judging an object to be inverted, while it retains its natural position with respect to all the objects around it. There is also a great difference between an object and its image, by means of which we become sensible of its existence and position. There seems to be no necessary connexion between their positions. The perception of things by sight seems to be no other than the remembrance of former perceptions acquired by our other senses, and we constantly conclude that the things which we now perceive will invariably affect us, in the same manner, as they formerly affected us in similar circumstances.

Now if the memory of the same sensations excited in the same part of the retina occasions the same judgment of the position of objects, the inverted images in the eye will answer this purpose as well as if they had been constantly erect. It is only necessary that the object and image should invariably preserve or change their situations and positions at the same time, according to some fixed and established law. And had a person been born with such an eye as would have erected the images of objects on the retina, he would talk and judge of objects just as other persons now judge and talk of them.

It has been also a matter of surprise to some, that an object should appear single to persons, whose eyes are in a sound state, where there is an image of the object formed in each eye. It is also from habit and experience, in a great measure, that we judge an object

to be single, provided the images are formed upon correspondent parts of the retina, which is always done, when the optic axes unite at the object. It is principally from experience that we learn to judge of external objects; as appears from persons who have been couched.

From Dr. Cheselden's account of persons couched by him, it appears, that, at first, they had no idea of figure, distance, or colour, by sight, until they had learned to see, as they would express it, or until they had the knowledge of these things by habit, and the assistance of their other senses. They all agreed in this, that they could not, at first, turn both their eyes to the same object, until they had acquired the power by habit; that objects at first seemed to touch their eyes, and that having much to learn, they would forget a thousand things which they had already learned. From experience and the assistance of other senses, we learn to account an object single, when its images in both eyes are formed on the correspondent parts of the retinas, that is, when the axes of the eyes meet at the object.

But when the axes of the eyes meet either between the eye and the object, or beyond it, the images do not then fall upon the correspondent parts, and the object will appear double. Any person may convince himself of the truth of this position, by holding up his finger before his eyes, while he is attentively looking at a more distant object, and his finger will at the same time appear double, as the images of it are not then formed on corresponding parts of the eyes; the optic axes uniting at the object, and not at the finger: but if he should unite them at the finger, by looking attentively at it, the more distant object will then appear

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