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lifting it up as you would raise it in a bucket, of which the piston formed the bottom. This common pump is, therefore, called the sucking and lifting pump, and is constructed on both these principles.
The forcing pump consists of a forcing power added to the sucking part of the pump. This additional power is exactly on the principle of the syringe; by raising the piston the water is drawn up into the pump, and by making it descend it is forced out. The large pipe A B, represents the sucking part of the pump, which differs from the lift- Hi ing pump only in its piston, P, being unfurnished with a valve, in consequence of which the water cannot rise above it. When, therefore, the piston descends, it shuts the valve, y, and forces the water (which has no other vent) into the pipe, D: this is likewise furnished with a valve, V, which, opening outwards, admits the water, but prevents its return. The water is thus first raised in the pump and then forced into the pipe, by the alternate ascending and descending motion of the piston, after a few strokes of the handle to fill the pipe, from whence the water issues at the spout.
LESSON XVII. ON THE STRUCTURE OF THE EYE. The body of the eye is of a spherical form. It has two membranous coverings ;—the external one, a a a, is called the sclerotica; this has a projection in that
part of the eye which is exposed to view, 6 b, which is called the cornea, because, when dried, it has nearly
the consistence of very fine horn, and is sufficiently transparent for the light to obtain free passage through it. The second membrane which lines the cornea, and envelopes the eye, is
called the choroid, this
a has an opening in front, just beneath the cornea, which forms the pupil, d, through which the rays of light pass into the eye. The pupil is surrounded by a coloured border of fibres, called the iris, 9 g, which by its motion, always preserves the pupil in a circular form, whether it be expanded in the dark, or contracted by a strong light.
The construction of the eye is so admirable, that it is capable of adapting itself more or less to the circumtances in which it is placed. In a faint light the pupil dilates so as to receive an additional quantity of rays; and in a strong light it contracts in order to prevent the intensity of the light from injuring the optic nerve. The eyes suffer pain when, from darkness, they suddenly come into a strong light; for the pupil being dilated, a quantity of rays rush in before it has time to contract. And when we go from a strong light into obscurity, we at first imagine ourselves in total darkness, for a sufficient number of rays cannot gain admittance into the contracted pupil to enable us to distinguish objects; but in a few minutes it dilates and we clearly perceive what was before invisible. The choroid, is imbued with a black liquor, which serves to absorb all the rays that are irregularly reflected, and to convert the body of the eye into a more perfect camera obscura. When the pupil is expanded to its utmost extent, it is capable of admitting ten times the quantity of light that it does when most contracted. In cats, and
animals which are said to see in the dark, the power of dilation and contraction of the pupil is still greater ; it is computed that their pupils may receive one hundred times more light at one time than at another. Within these coverings of the eye-ball are contained three transparent substances called humours. The first occupies the space immediately behind the cornea, and is called the aqueous humour, from its liquidity and its resemblance to water. Beyond this is situated the crystalline humour, cc, which derives its name from its clearness and transparency; it has the form of a lens, and refracts the rays of light in a greater degree of perfection than any that have been constructed by art; it is attached by fibres, to each side of the choroid. The back part of the eye, between the crystalline humour and the retina, is filled by the vitreous humour, V v, which derives its name from a resemblance it is supposed to bear to glass or vitrified substances. The membranous coverings of the eye are intended chiefly for the preservation of the retina, rr, which is by far the most important part of the eye, as it is that which receives the impression of the objects of sight. The retina consists of an expansion of the optic nerve, of perfect whiteness ; it proceeds from the brain, enters the eye at o, on the side next the nose, and is finely 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 other point of an object, in a corresponding point on the retina, and the image is thus rendered distinct and strong.
Lesson XVIII. 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 retinas From this focus the rays proceed diverging, and consequently form a very confused image on the retina. This is the defect of shortsighted 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 shortsighted 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 dis
tant 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 object 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 eye, because the rays that fall on the crystalline humour are too divergent to be refracted to a focus on the retina. The confusion, therefore, arising from viewing an object too 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.