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enters the eye at n on the side next the nose, and is finally spread over the interior surface of the choroid. The rays of light which enter the eye by the pupil, are refracted by the several humours in their passage through them, and unite in a focus on the retina.

Rays proceed from bodies in all possible directions. We must, therefore, consider every part of an object which sends rays to our eyes as points from which the rays diverge, as from a centre. Divergent rays, on entering the pupil, do not cross each other; the pupil, however, is sufficiently large to admit a small pencil of them; and these, if not refracted to a focus by the humours, would continue diverging after they had passed the pupil, would fall dispersed upon the retina, and thus the image of a single point would be expanded over a large portion of the retina. The divergent rays from every other point of the object would be spread over a similar extent of space, and would interfere and be confounded with the first, so that no distinct image could be formed on the retina. The refraction of the several humours unites the whole of a pencil of rays, proceeding from any one point of an object, in a corresponding point on the retina, and the image is thus rendered distinct and strong.

That imperfection of sight which arises from the eyes being too prominent, is owing to the crystalline humour being too convex; in consequence of which it refracts the rays too much, and collects them into a focus, before they reach the retina. From this focus, the rays proceed diverging, and consequently form a very confused image on the retina. This is the defect of short-sighted people; and it is remedied by bringing the object nearer to the eye; for the nearer an object is brought to the eye the more divergent the rays fall upon the crystalline humour, and consequently do not so soon converge to a focus. This focus, therefore, either falls upon the retina, or at least approaches nearer to it, and the object is proportionally distinct. The nearer, therefore, an object is brought to the crystalline or to a lens, the further the image recedes behind it. But short-sighted persons

have another resource for objects which they cannot permit to approach their eyes. This is to place a concave lens before the eye, in order to increase the divergence of the rays, the effect of a concave lens, being exactly the reverse of a convex one. By the assistance of such glasses, therefore, the rays from a distant object fall on the pupil as divergent as those from a less distant object; and, with short-sighted people, they throw the image of a distant object back as far as the retina. Those who suffer from the crystalline humour being too flat, apply an opposite remedy: that is to say, a convex lens to make up for the deficiency of convexity of the crystalline humour. Thus elderly people, the humours of whose eyes are decayed by age, are under the necessity of using convex spectacles; and when deprived of that resource, they hold the objects at a distance from their eyes, for the more distant the object is from the crystalline, the nearer the image will be to it. These two opposite defects are easily comprehended; but the greatest difficulty remains, namely, how any sight can be perfect; for, if the crystalline humour be of a proper degree of convexity to bring the image of distant objects to a focus on the retina, it will not represent near objects distinctly; and if, on the contrary, it be adapted to give a clear image of near objects, it will produce a very imperfect one of distant objects. Now to obviate this difficulty, and adapt the eye either to near or to distant objects, power is given to us to increase or diminish in some degree the convexity of the crystalline humour, and also to project it towards, or draw it back from the object, as circumstances require. In a young, well-constructed eye, the fibres to which the crystalline humour is attached, have so perfect a command over it, that the focus of the rays constantly falls on the retina, and an image is formed equally distinct both of distant objects and of those which are near. We cannot, however, see an object distinctly if we bring it very near to the because the rays fall on the crystalline humour too divergent to be refracted to a focus on the retina. The confusion, therefore, arising from viewing an object too

eye,

near the eye, is similar to that which proceeds from a flattened crystalline humour; the rays reach the retina before they are collected to a focus.

We conclude this subject with the following beautiful observations on the eye, from the pen of Addison :

Our sight is the most perfect and most delightful of all our senses. It fills the mind with the largest variety of ideas; converses with its object 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, com. prehends the largest figures, and brings within our reach some of the most remote parts of the universe.

It is this sense which furnishes the imagination with its ideas. We cannot, indeed, have a single image in the fancy that did not make its first entrance through the sight; but we have the power of retaining, altering, and compounding those images, which we have once received, into all the varieties of picture and vision that are most agreeable to the imagination; for by this faculty a man in a dungeon is capable of entertaining himself with scenes and landscapes more beautiful than can be found in the whole compass of nature.

A beautiful prospect delights the soul, as much as a demonstration; and a description in Homer has charmed more readers than a chapter of Aristotle. Besides, the pleasures of the imagination have this advantage, above those of the understanding, that they are more obvious, and more easy to be acquired. It is but opening the eye, and the scene enters. The colours paint themselves on the fancy with very little attention of thought or application of mind in the beholder. We are struck, we know not how, with the symmetry of any thing we see,

and immediately assent to the beauty of an object, without inquiring into the particular causes and occasions of it.

A man of polite imagination is let into a great many pleasures, that the vulgar are not capable of receiving. He can converse with a picture, and find an agreeable companion in a statue. He meets with a secret refreshment in a description, and often feels a greater satisfaction in the prospect of fields and meadows, than another does in the possession. It gives him, indeed, a kind of property in every thing he sees, and makes the most uncultivated parts of nature administer to his pleasures ; so that he looks upon the world, as it were, in another light, and discovers in it a multitude of charms, that conceal themselves from the generality of mankind.

SECTION V.

INTRODUCTION TO CHEMISTRY.

CHEMISTRY is the science which makes known to us the nature and properties of all bodies, whether these bodies be simple or compound-solid, liquid, or

aeriform.

The importance of the science of chemistry is evident from the following considerations. In acquiring a knowledge of the constitution of the atmosphere, in investigating the changes to which it is subject, the variations of temperature, the laws of winds, dew, rain, hail, and snow, chemistry is our principal, our only satisfactory guide. These remarkable changes-changes, which, because familiar, do not produce any emotion in the mind, though in themselves truly wonderful-are chemical operations on a magnificent scale, and can only be explained on chemical laws.

In examining the various objects which compose the mineral, vegetable, and animal kingdoms, chemistry is essentially requisite for the successful prosecution of our inquiries.

In the art of extracting metals from their ores, in purifying and combining them with each other, almost all the processes are purely chemical. The arts of glass and porcelain-making-of tanning, soap-making, dying, and bleaching-depend entirely upon chemistry; and all the processes of baking, brewing and distilling, and most of the culinary arts, are chemical operations.

The transformations of chemistry, by which we are enabled to convert materials apparently useless into important objects of the arts, are opening up every day

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