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will appear to your view, though it has remained exactly where it was.

a

Fig. 2.

This proves that when a ray of light (see the line a b in the figure) passes from water into air, or the contrary, it is bent or refracted.

[graphic][subsumed][merged small]

In fig. 3, the line a b, leading from the shilling to the eye, is bent or refracted at s, the surface of the water.

Now,

as we naturally think that every ray of light is straight, we imagine that the ray b s has proceeded from a1; and so we conclude that the shilling has risen from a to a'.

It has been said above that a ray of light in passing from water into air is bent or refracted; but there is an exception to this general rule. If you dip an oar into the water in a slanting direction, it will seem broken on account of this refraction, but if you hold it perfectly upright it will not seem broken.

Any substance through which light can pass is called a

Fig. 4.

medium, and different media have different densities. glass is denser than water, and water is denser than air. We can now state the law of refraction.

Thus

When a ray of light, passing through one medium, meets another medium of different density in a slanting direction, it is refracted or bent. Thus a ray of light passing

through water into air in a slanting direction is bent at the surface of the water (fig. 3). Thus, again, when a ray of light passes through a pane of glass in a slanting direction, it is twice refracted-viz., when it enters and when it leaves the glass (fig. 4).

Similarly, a ray which passes through three panes of glass will be six times refracted.

If the ray does not fall in a slanting direction, there is no refraction.

LENSES.-VISION.

A LENS is a circular piece of glass having one or both sides curved either inwards or outwards. There are six kinds of lenses, the most useful of which is the double-convex lens (see fig. 1).

F

Fig. 1.

A lens refracts all rays which pass through it except that which goes through its centre. In most cases there are two refractions-one on entering the lens, and another on leaving it at the other side.

Let us now see what effect a double-convex lens will have on a number of parallel rays of light. With the exception of one passing through the centre of the lens, they must all be refracted twice, and it will be found that they meet in a point (F) on the other side of the lens.

This point F is called the Principal Focus of the lens. There is another principal focus on the left side of the lens. It is marked by a dot, and is the same distance on the left as F is on the right of the lens. (The principal foci are shown in succeeding figures by dots.)

Let us now see what will be the effect of a pencil of rays meeting a lens like that in fig. 1 above.

meeting a lens like that in Fig. 1. If the pencil of rays proceeds from the point P, beyond the principal focus on the left, the rays will be refracted and brought to a focus (ƒ) beyond the principal focus on the other side of the lens.

P

Fig. 2.

It will be convenient for us, when we want to show how an object is viewed, to draw just two pencils of rays from

Fig. 3.

the extremities of that object. Thus let fig. 3 represent an eye beholding an object. From each end of the arrow (the object looked at) we have a pencil of rays to the eye, each pencil consisting of two or three rays. Such is the plan we are obliged to follow, for it would be quite impossible to draw all the pencils and all the rays, for they are innumerable.

The eye is a wonderful instrument. It may be briefly described as a small round chamber, having a blank wall at the back called the retina, and a window in front bulging out like the glass of a watch. The eyelids form a sort of

outside shutter for this window; whilst within the eye hangs a sort of coloured curtain to the window, called the iris, having a hole in the centre known as the pupil. Behind the pupil, through which light enters the eye, is a doubleconvex lens, not of glass, but of some substance which serves the same purpose. It is the function of the lens to refract the rays from any object looked at, so as to form an image of the object on the retina.

Let us now draw the same two pencils of rays as in fig. 3, and see what becomes of them when they pass into the eye. The lower pencil is drawn with dotted lines for the sake of clearness.

Fig. 4.

The pencils have been refracted by the lens in the eye, have crossed each other, and a small picture of the arrow is seen on the retina upside down.

For the purpose of distinct vision it is necessary that all the rays from the same point of an object should come to a focus, or meet in one point, on the retina. If a person's eyesight is imperfect, the image is formed a little in front of the retina, or a little behind it, according as a person is "near-sighted" or the reverse. When this is the case, the proper remedy is the use of a couple of lenses, called a pair of spectacles, to enable the eyes to do their duty properly.

But when the eye has no defect, it has in itself the power of altering its own lens, according as it is looking at a distant or a near object, so as to throw the focus always on the retina.

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