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passes out of a rarer into a denser medium, it is drawn to the perpendicular; thus if s a pass from air into glass, it moves, in its passage through it, in the line a x, which is nearer to the perpendicular a b than the line a s, which was its first direction.

But when a ray passes from a denser medium into a rarer, it moves in a direction farther from the perpendicular; thus if the ray x a pass through glass or water into air, it will not, when it comes to a, move in the direction a m, but in the line a s, which is further than a m from the perpendicular a P.

James. Can you show us any experiment in proof of this?

Tutor. Yes, I can here is a common earthen pan, on the bottom of which I will lay a shilling, and will fasten it with a piece of soft wax, so that it shall not move from its place, while I pour in some water. Stand back till you just lose sight of the shilling.

James. The side of the pan now completely hides the sight of the money from

me.

Tutor. I will pour in a pitcher of clear

water.

James. I now see the shilling: how is this to be explained ?

1

Tutor. Look to the last figure, and conceive your eye to be at s, a b the side of the pan, and the piece of money to be at x: now when the pan is empty, the rays of light flow from x, in the direction x am, but your eye is at s, of course you cannot see any thing by the ray proceeding along x am. As soon as I put the water into the vessel, the rays of light proceed from x to a, but there they enter from a denser to a rarer medium; and therefore, instead of moving in a m, as they did when there was no water, they will be bent from the perpendicular, and will come to your eye at s, as if the shilling were situate at n.

James. And it does appear to me to be

at n.

Tutor. Remember what I am going to tell you, for it is a sort of axiom in optics: "We see every thing in the direction of that line in which the rays approach us last.”

Which may be thus illustrated: I place a candle before the looking-glass, and if you stand also before the glass, the image of the candle appears behind it; but if another looking-glass be so placed as to receive the reflected rays of the candle, and you stand before this second glass, the candle will appear behind that; because the mind transfers every object seen along the line in which the rays came to the eye last.

Charles. If the shilling were not moved by the pouring in of the water, I do not understand how we could see it afterwards.

Tutor. But you do see it now at the point n, or rather at the little dot just above it, which is an inch or two from the place where it was fastened at the bottom, and from which, you may convince yourself, it has not moved.

fames. I should like to be convinced of this will you make the experiment again, that I may be satisfied of it?

Tutor. You may make it as often as you please, and the effect will always be the same; but you must not imagine that the

shilling only will appear to move, the bottom of the vessel seems also to change its place. James. It appears to me to be raised higher as the water is poured in.

Tutor. I trust you are satisfied by this experiment: but I can show you another equally convincing; but for this we stand in need of the sun.

Take an empty vessel A, a common pan or basin will answer the purpose, (Plate 1. Fig. 3.) into a dark room, having only a very small hole in the window shutter: so place the basin that a ray of light s s shall fall upon the bottom of it at a, here I make a small mark, and then fill the basin with water. Now where does the ray fall? James. Much nearer to the side at b. Tutor. I did not move the basin, and therefore could have had no power in altering the course of light.

Charles. It is very clear that the ray was refracted by the water at s: and I see that the effect of refraction in this instance has been to draw the ray nearer to a perpendicular, which may be conceived to be the side of the vessel.

Tutor. The same thing may be shown with a candle in a room otherwise dark : let it stand in such a manner as that the shaddow of the side of a pan or box may fall somewhere at the bottom of it; mark the place, and pour in water, and the shadow will not then fall so far from the side.

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