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breaks up, and the disconsolate knight finds himself on a barren heath, or in a solitary desart. It is not improbable but something like this may be the state of the soul after its first separation, in respect to the images it will receive from matter; although, indeed, the ideas of colours are so pleasing and beau tiful in the imagination, that it is possible the soul will not be deprived of them, but perhaps find them excited by some other occasional cause, as they are, at present, by the different impressions of the subtle matter on the organs of sight.

These reflections, which discover such a delicate sensibility to the beauty of nature, and which contain such a happy illustration of the author's idea, are grounded upon the doctrine of Mr. Locke, that light and colours, as apprehended by the imagination, are only ideas in the mind, and not qualities that have any existence in matter.

No. XLVI.

ON VISION.

With thought from prepossession free, reflect
On solar rays, as they the light respect.
The beams of light had been in vain displayed,
Had not the eye been fit for vision made:
In vain the Author had the eye prepared
With so much skill, had not the light appeared.

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LUCRETIUS is the only bard I can recollect who has ornamented the philosophy of vision with

poetical illustration. But his theory, which was that of his master Epicurus, however beautiful in poetry, has long vanished before the accurate researches and superior attainments of the moderns.

If the construction of the universe were not so evident a proof of the existence of a supremely wise and benevolent Creator, as to render particular arguments unnecessary, the structure of the eye might be offered as one, by no means the least. This instance, among numberless others, demonstrating, that the most exquisite performances of art are infinitely short of those which are continually produced by the Divine Mechanic.

Of all the senses, the sight is that which furnishes the soul with the quickest, most extensive, and most varied perceptions. It is the fertile source of the richest treasures of imagination, and it is to that principally, that the soul owes the ideas of beauty, and of that varied unity which enchants it.

Our sight (as observed by an admirable writer) is the most perfect and delightful of all our senses. It fills the mind with the largest variety of ideas, converses with its objects at the greatest distance, and continues the longest in action, without being tired or satiated with its proper enjoyments. The sense of feeling can indeed give us a notion of extension, shape, and all other ideas that enter at the eye, except colours; but, at the same time, it is very much straitened and confined in its operations, to the number, bulk, and distance of its particular objects. Our sight seems designed to supply all these defects, and may be considered as a more delicate and diffusive kind of touch, that spreads itself over an infinite multitude of bodies, comprehends the largest figures, and brings into our reach some of the most remote parts of the universe.'

Night has gradually withdrawn her sable curtain from the face of the earth. Aurora proclaims the

approach of the glorious sun. He appears, and all nature seems to have received a new creation. What majesty! what splendour! what light! what colours!

But by what secret mechanism are my eyes made capable of communicating to me such lively, varied, and infinite perceptions? How do I discover with so much facility and quickness every object that surrounds me?

The eye is composed of several tunics, or coats, one within the other, and is filled within with transparent humours of different refractive densities. The external tunic, called the sclerotica, is white on the anterior part, except a circular portion immediately in front, which is transparent, and more convex than the rest of the eye: this transparent part is called the cornea. Immediately adherent to the sclerotica within, is the choroides, or uvea, which, at the circumference of the cornea, becomes the iris, being expanded over great part of its surface, though not contiguous to it. The iris is composed of two kinds of muscular fibres; the one softened, like the radii of a circle, toward its centre, and the others form a number of concentric circles round the same centre. The central part of the iris is perforated, and the orifice, which is called the pupil, is of no constant magnitude; for, when a very luminous object is viewed, the circular fibres of the iris contract, and diminish its orifice; and, on the other hand, when objects are dark and obscure, the radial fibres of the iris contract, and enlarge the pupil, so as to admit a greater quantity of light into the eye. The iris is variously coloured in different persons, but according to no certain rule. In general, they whose hair and complexion are light-coloured, have the iris blue or gray; and, on the contrary, they whose hair and complexion are dark, have the iris of a deep brown. But what specific difference this

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may occasion in the sense, or whether any at all, is not discoverable. Within the uvea is another membrane, which, at the circumference of the cornea, becomes fibrous, and is called the ligamentum ciliare. This ligature is attached to the circumference of a double convex lens, whose axis corresponds with the centre of the pupil, and which, by means of the fibres, can be altered, in a small degree, in position, and perhaps in figure. This lens is termed the crystalline humour; and is included in a very strong and transparent membrane called the arachnoides. Between the crystalline humour and the cornea is contained a clear transparent fluid, called the aqueous humour; and between the crystalline humour and the posterior part or bottom of the eye, is included another clear transparent fluid which is called the vitreous humour. The refractive density of the crystalline is greater than those of the humours that surround it. On the side next to the nose, a nerve is inserted in the bottom of each eye, about twentyfive degrees from the axis of the crystalline, which, after entering the eye, is spread into an exceeding fine coat of network, termed the retina. Lastly, a very black mucus or slime, is spread over all the internal parts of the eye, that are not transparent, except the anterior part of the iris, which, as before observed, is coloured.

From this account of the structure of the eye, I shall proceed to that of vision, or the act of perceiving external objects by the organs of sight. And this is well defined to be a sensation, by which, from a certain motion of the optic nerve, made in the bottom of the eye by the rays of light emitted or reflected from objects, and hence conveyed to the common sensory in the brain, the mind perceives the luminous object, its quantity, quality, figure, &c. And thus, all the modern philosophers, whether Cartesians or Newtonians, agree, that vision is per

formed by rays of light, reflected from the several points of objects received in at the pupil, refracted and collected in their passage, through the coats and humours before-mentioned, to the retina; and thus striking, or making an impression, on so many points of it; which impression is conveyed to the brain, &c. by the correspondent capillaments, or fibres, of the optic nerve.

The cornea, or second coat of the eye, being of a convex figure, performs the office of a glass lens. To illustrate this by a familiar example, put a glass lens into a hole made in the window-shutter of a dark room; present a pasteboard to this lens at a uitable distance; and you will immediately have a picture, in which all the objects without will be painted to the greatest precision, and according to all the rules of the most exact perspective. It will be like a moving picture whenever these objects move. You will then see rivers rushing down from the tops of mountains, and meandering in the plains; birds hovering in the air; fishes sporting on the surface of the water; flocks frisking in the meadows; and, in fine, all the possible varieties of prospect.

Substitute instead of the lens the real eye of an ox, newly stript of its teguments, and you will see the same picture on the membrane which covers the bottom of it; but all the figures will be painted in a much smaller size. You can never be tired of admiring the extreme delicacy of this piece of miniature, and will be astonished to behold a field, you fifteen or twenty miles square, exactly delineated on a piece of vellum of a few lines in length.

The structure of the eye of an ox is the same, with respect to the essential parts of it, as that of ours; and thus the mechanism of vision is explained. The humours of the eye are the lens of the camera obscura; the retina is the pasteboard.

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