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rays converge after refraction, more of them will enter the eye; and confused, because the eye is nearer to the lens than the place of the distinct picture. It will also appear magnified, in the inverse ratio of the distance of the eye from the place of the distinct picture. So that as the eye removes from the lens to the place of the distinct picture, the apparent magnitude of the object will continually increase, until it become infinite and confused, when the eye arrives at that place; afterwards it will become inverted, confused and diminished, when the eye removes to a greater distance. It will then also become fainter, because the rays now diverging from the place of the distinct picture, fewer of them will enter the eye. If the eye be very close to the place of the distinct picture or focus, it will see the object as confusedly, as if it had been close to the object; since the rays diverge from it, as they do from the object; but when it recedes farther, it will see it more distinctly...

When the eye and object are fixed in their stations, and not farther apart than the place of the distinct picture, wheresoever the convex lens may be between them, as the lens moves from the eye towards the ob. ject, its apparent diameter increases until it arrive at the middle point, and then it begins to decrease till the lens arrive at the object, where its apparent diameter is not magnified, any more than when the lens was close, to the eye.

When the eye is in the principal focus of a convex lens, the apparent diameter of an object is not altered by the motion of the object from or towards the lens; but the removal of the object increases its apparent diameter, when the eye is more remote, and diminishes it, when the eye is nearer. When the eye is in the

focus, as the object departs from the lens, its last image departs with it, and thereby grows less, but at the same time the place of the distinct image is continually approaching to the place of the eye, which makes the apparent diameter of the object to increase, and therefore as it increases as fast on one account as it is diminished on another, it will continue unaltered. That the increase and decrease of the apparent diameter of the object is just equal in this case is plain, because it cannot be infinitely diminished till the object removes to an infinite distance, when the place of the distant picture comes to the eye, and when it sees the object infinitely great. But when the eye is farther from the lens than the focus, the apparent diameter of the object increases faster than it is diminished, and therefore increases upon the whole, and vice versa.

If the object be at a greater distance than the principal focus of a convex lens, and the eye be nearer than the place of the distinct picture, then, if the lens be moved from the right hand to the left, the object appears to move the contrary way, but the same way, if the lens be fixed and the eye be moved. Every thing is reversed, if the eye be farther from the lens, than the place of the distinct picture. For the object will then appear to move the same way with the lens, and contrary to the motion of the eye. By the motion of the lens, the rays fall with different obliquities, and are differently refracted; and by the motion of the eye, different refracted rays come to it, so that the object must appear to change its position continually.

CONCAVE LENSES.

WHEN an object is seen through a concave lens, it seems fainter, as the rays diverge and fewer enter the eye; it appears erect, as the rays do not cross each other; but it will appear confused to all such eyes as are not very convex: Yet it may be made more distinct either by removing the eye to a moderate distance, or by making the rays pass through a small hole, so that the most diverging rays may be cut off, and only the middle rays, which diverge least, be suffered to enter the eye.

The apparent magnitude of the object is diminished, as the imaginary radiants are nearer to the lens than the object, and both are seen under the same angle from the lens; and it decreases with the departure of the object; because the last image or imaginary radiant removes with it, and consequently is seen under a less angle.

If the eye and object be fixed in their places, as the concave lens recedes from the eye, the apparent magnitude of the object decreases, until it arrive at the middle station between the eye and object, and then it begins to increase until the lens arrive at the object. For the apparent magnitude of the object is not altered when the lens is at either extreme, and therefore it must be least in the middle.

If the concave lens be moved from the right hand to the left, the object. appears to move the same way with the lens; but if the eye be moved, it appears to move in a contrary direction.

Purblind persons, whose

eyes are very convex, see distinctly through concave lenses; while old persons, whose eyes are flatter, through the wasting of the hu

mours of the eye, see more distinctly through convex lenses; as we shall see more fully, when we consider

the eye.

Objects seen through a plane thick glass appear erect, nearer, brighter and larger, than to the naked eye: and they do not seem to move by the motion of the eye. The imaginary radiant is nearer to the glass than the object, and therefore it will appear magnified as nearer objects appear; more rays will enter the eye to make it appear brighter, and it must appear erect, as the rays do not cross each other: the motion of the eye does not alter the obliquity of the incident rays, nor does the motion of the glass; and therefore the object must appear to be stationary, however the eye or glass may move.

Objects seen through a polygonous glass, with many surfaces inclined to each other, appear to be multiplied, as the rays that fall upon each of the surfaces will be refracted in different directions, and consequently there will be as many images formed, as there are surfaces to refract the rays to the eye.

THE EYE AND VISION.

HAVING Considered how the rays of light passing through a lens are collected into a focus, and there form images of objects; it will be easy to understand, how vision is effected, by the rays that issue from any object, and are converged by the refractive power of the various humours of the eye to innumerable points on the bottom of the eye, where the images of external objects are formed, and by their impressions on the optic nerve make us sensible of the existence of these objects. For the humours are in reality so many lenses, disposed and combined together in such a man

ner, as to converge the rays of light to a focus on the bottom of the eye, where the image of the external object is completely and accurately formed, when the eye is in a proper state for effecting vision. A complete description of the mechanism of the eye belongs to anatomists to give; but that you may understand how vision is performed by it, we shall give you so much of the general description as will be essential to our present purpose.

The human eye is nearly globular, but not perfectly so, consisting of two segments of different spheres, that which forms the anterior part being the lesser. It consists of three humours of different refractive powers, inclosed in as many coats. The outward coat, which covers the larger segment of the eye, is called the sclerotica, being of a hard, white, and opake substance, as is signified by the name. The continuation of this coat, where it covers the fore part of the eye, is denominated the cornea, from its resemblance of horn in its consistence and transparency. This being convex and transparent serves to converge the rays of light that fall upon it to a focus, at a small distance beyond the bottom of the eye, could they proceed so far. Next within this lies the choroides, of a blackish colour, which serves for a lining to the sclerotica, with which it is connected while it covers the back part of the eye. But in the anterior segment, it is separated from the cornea, and floats in the aqueous humour, where it assumes the name of the iris or uvea, from the different colours which it has in different eyes. It is composed of circular and radial fibres, by the alternate contraction or dilatation of which, the central hole in the middle of it, called the pupil, is lessened or enlarged, to regulate the quantity of light, that is found to

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