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Such are spectacle-glasses, called lenses.
568. The eye consists of a transparent horny coat on its outside, called the cornea; within it, is a pure liquid called the aqueous humour ; and within the aqueous humour, is a lens, like a spectacle-glass, called the crystalline humour.
Beyond that, is a jelly-like humour, called the vitreous humour, filling the ball of the eye; and at the back of the eye, is spread the optic nerve, retina, or fine net-work, to receive the impressions of the rays of light.
The horny coat, the lens called the chrystalline humour, and the other transparent humours, answer the general purpose of one spectacle-glass, with nice and wonderful powers of adaptation.
The Circlerepresents the ball of the eye; and the arrow A B C, any object to be seen by the eye. Rays proceed from all points of the arrow; but some of those from A B C only, are represented, to avoid confusion. These flow from A B C in straight lines as represented. They fall on the cornea at E F, pass through the aqueous hu. mour to the iris or pupil; thence, through the convex lens or chrystalline humour, shaded dark; and thence, into the ball of the eye, filled with the vitreous humour, reconverging and producing a perfect picture of the arrow
at the back of the eye at c, b, a, where is spread the fine net-work, or retina of the optic nerve. The tube at the corner, is the optic nerve going to the brain :-such is the simple, but wonderful economy of vision!
Obs.-To comprehend the effect on the lens of the eye in producing vision in the different ways in which the rays may fall upon it, holda spectacle-glass, by means of a rule or stick, at unequal distances from a wall; which wall may be supposed to represent the back of the eye. Then place a candle at such distance, or adapt the lens to that distance, and a beautiful picture of the candle reversed will be seen upon the wall. Keep the lens fixed, and move the candle nearer to it, and the image will be dess distinct, and quite vanish, as it approaches; then carry the candle backward, to a greater distance than the first distance, and, in like manner, the image will again become indistinct. In the first instance, the rays fall with such a degree of obliquity, as, when operated upon by the refraction of the glass, occasion the whole of them to converge, and reproduce, in opposite and cross-directions, an image of every part of the candle; but when carried nearer, the refracting power of the glass was unequal to the great degree of convergency required, and either the image would be produced at a greater distance, that is, beyond the wall, or the rays would go out parallel; or even diverge or spread, if the candle were carried still nearer. Hence, vision depends on the parallelism or obliquity of the rays proceeding from an object, and on the power of the eye to accommodate itself to that obliquity; and when that is greater than the power, art is necessary to diminish or increase the obliquity of the rays, so as to accommodate them to the powers of the eye. Aged people require spectacles to increase the convergency; because in these, all the humours diminish, and the chrystalline lens of the eye becomes flatter, and the cornea itself less convex: hence, the power of convergency is diminished, and the images of objects fall beyond or behind the optic nerves. This can be illustrated by having two lenses of different con
vexities held at the same distance from the wall; and it will be proved, that when the more convex, or youthful lens, produces a distinct picture, the flatter, or aged one, produces a confused image.
569. The different distances, at which lenses produce on a wall the representation of objects, is called their focal distance; and it is the centre of the circle, of which the surface of the common double lens is a part.
The concave lens has the opposite effect; it diverges or spreads the rays, instead of converging them to a focus. Hence, when the eye is too flat in old age,
the convex lens helps its converging powers.
And when it is too convex, as in short-sighted people, the concave lens counteracts the convexity of the eye, spreads the rays, and renders vision distinct.
Obs. 1.–The next circle represents the ball of an aged eye; in which, owing to the decay of the humours, the cornea is not convex enough to converge the rays on the retina, but only a little beyond it. The object will, therefore, appear with a burr around it, or confused. If, then, a convex lens or spectacle-glass be interposed, as a, b, this will give a converging direction to the rays be. fore they reach the eye; and, of course, its converging power will then be sufficient to produce the figure exactly on the optic nerve.
THE CONVEX LENS FOR AGED PERSONS.
Obs. 2.-In this figure, the cornea is supposed to be too convex, as in short-sighted persons; and the rays are converged before they reach the back of the eye or retina. The
eye, in this case, performs too much ; and more is given it to perform, by interposing a concave lens, so that the rays, instead of falling parallel on the eye, may actually fall divergent.
THE CONCAVE LENS FOR SHORT-SIGHTED PERSONS.
570. But besides the useful invention of spectacles, arising from this power of converging rays of light in convex lenses, a very important use arises from their combination in microscopes and telescopes, the principle of which is exceedingly simple.
Every visible object is of a visible size, proportioned to the angle which it makes to the eye; and that angle is also always in the inverse proportion * of the distance, which the eye is from the object.
Obs.To prove this, try a simple microscope without any glass, and it will enable you to see an object clearly at the dis; knce of an inch ; which, with the naked eye,
* Tlie term inverse signifies something like a contrary. Thus the size is not as the distance ; because, as the distance is greater, the size is less ; the proportion is, therefore, not direct, but opposite, contrary, or inverse.
could not be well seen at less than eight inches; and the object, suppose a grain of sand, will apparently be eight times larger in diameter, 64 times in surface, and 512 times larger in bulk! This simple microscope is nothing more, thana small hole pricked with a fine needle through a piece of blacked card. The hole, by limiting the number of rays from the object, will enable you to see the object as above, and prove that the size is as the angle, or inversely as the distance. Every object in length or breadth, is in size, in the inverse proportion of its distance; because the angle, which its size subtends to the eye, is of a size inversely proportioned to the distance. Thus, a man at 100 yards distant, is but half the apparent. height that he is at the distance of 50 yards, and only a tenth of his size at ten yards distance. He is magnified, therefore, ten times, by any contrivance which enables us to view him under the same angle at 100 yards distant as we should see him with the naked eye at 10 yards.
571. The object, then, of all arrangements of glasses, or lens, in microscopes and telescopes, is first to produce an image of the object, and then to dispose of the rays proceeding or diverging from the image, in such manner, as that it may produce distinct vision in the eye.
The sole use of the object-glass is so to dispose the rays anew, as that they may produce vision by the approximation of an eye-glass; and the magnifying power will depend on the closeness with which the eye-glass enables the eye to see the image produced by the object-glass; or, on the convexity of the eye-glass.
Or, in other words, the magnifying wer will be in the ratio of the focal distance of the objectglass to that of the eye-glass.
Obs. 1.-If a tree, at the distance of 400 yards, subtend -an angle of one degree to the naked eye, and an image of it is produced by the object-glass of a telescope, and