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the water. Such, too, is the effect of refraction on rays of light; which, on passing into water, or any heavier transparent body, are bent downward.

Obs. - To verify and understand this principle, put a halfpenny into an empty basin, and stand at such distance that the coin may not be visible; then, let another personr pour pure water into the basin, and the halfpenny will become visible: this arises from the bending of the rays iu their passage into air at the surface of the water.

576. Hence, when the rays of light coming from the celestial bodies, arrive at our atmosphere, they are bent downward ; and those bodies appear, when in the horizon, sa degree higher than they are.

Many rays of light are reflected at the surface of a new transparent medium, in an angle equal to that in which they fall on the surface, and on this principle all mirrors are constructed.

577. Advantage is taken by man of the property of refraction, to construct new mediums of such shape, as that all rays that fall on them may, on coming out of them, converge in one point instead of going straight forward.

The construction of surface which produces this effect is the convex; and all rays of light which fall on a circular surface of glass, &c., are converged on the other side into a series of corresponding points, representing the objects whence the light proceeded.

Such are spectacle-glasses, called lenses.

Obs. Two or three lenses, of different degrees of convexity, may be purchased at any optician's for sixpence

pachand the varlous Illustrative experiments that may be tried with them, render it desirable that they should be at hand, in every seininary of education.

670. The eye consists of a transparent horny coat on its outside, called the cornca; 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 tlue back of the eye, is spread the optio nerve, retina, or fine net-work, to receive the impressions of the rays of light.

The horny coat, the lens, called the crystal line bumour, and the other transparent hu mnours, answer the general purpose of one spec. tacle-glass, with nice sod wonderful powers of adaptation.

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The Curele represents the ball of the eye and the Arrow ABU, any olject to be seen by the eye. Ray, proceed from all points of the arrow, but some of those from

A B C only, are represented, in avoid confusion. These flow from A B o in straight lines as represented. They fall on the corner at 1 1, pasg through the aqueous humour to the fris or pripit, thence, Ibrough the convex line or crystalline humour, shaded dark, aud tliedce, 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, hold a 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 auch 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 less 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, occasions 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 is unequal to the great degree of convergency required, and either the image will be produced at a greater distance, that is, beyond the wall, or the rays will go out parallel ; or even diverge or spread, if the candle be carried till nearer, Hence, vision depends on the parallelisın or obliquity of the rays proceeding from an object, and on the power of the cye to accommodate itself to that obliquity ; and when that is greater than the power, art is necessary to dimi. nish 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 convergeris, because in these, all the humours diminish, and the chrystalline lens of the eye becomes flatter, and the cornea itself lens couvex: hence, the power of convergency is diminished, and the images of objects fall beyond or behind the optic nerves. This can be illustrated in a moment, by having two lenses of different convexities held at the same dis

tance from the wall, and it will be proved, that when the more couvex, or youthfal lens, produces a distinct picture, the latter, or aged one, produces A confused

579. 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 lias the opposite effect; it divergre or spreads the rays, instead of converging them to a focus.

Jlence, when the eye is too fat in old age, the convex lens' lielps its converging powers.

And when it is too convex, as in short-sighted people, the concave lens counteracts the con vexity of the eye, spreads the rays, and renders vision distimet,

Obs, 1,--The next circle represents the ball of an aged eye , in which, owing to the decay of the humours, the Cornem 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 confined Il, then, a convex lens or spectacle glass be interpozeif, as a, b, this will give a converging direction to the main before they reaclitli yer and, of course, its converging power will then be suffcient to produce the figure es Metly on the oplie nerve,

TITE CONVIX LENS FOR AGED PERSON»,

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Obs. 9. 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.

(TRE CONCAVE LENS FOR SHORT-SIGHTED PERSONS.

580. But besides the useful invention of spertacles, 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 distance of an inch ; which, with the naked eye, could not be well seen at less than eight inches; and the

• The term invorse signifies something like a contrary. Thas the size is not as the distance ; because, as the distance is greater, the size is loss; the proportion is, tbera. fore, noi direct, but opposite, contrary, or invers.

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