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, 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.

James. 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 Now where does the

with water.

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 the 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

1

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 other wise dark: let it stand in such a manner as that the shadow 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.

CONVERSATION IV.

Of the Reflection and Refraction of Light.

TUTOR. We will proceed to some farther illustrations of the laws of reflection and refraction. We shut out

all the light except the ray that comes in at the small hole in the shutter: at the bottom of this basin, where the ray of light falls, I lay this piece of looking-glass; and if the water be rendered in a small degree opaque by