reflected ray, is called the angle of reflection.

James. Are these in all cases equal, shoot the marble as you will?

Tutor. They are: and the truth holds equally with the rays of light:both of you stand in front of the looking-glass. You see yourselves, and one another also; for the rays of light flow from you to the glass, and are reflected back again in the same lines. Now both of you stand on

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one side of the room. What do you see?

Charles. Not ourselves, but the furniture on the opposite side. Tutor. The reason of this is, that

the

rays of light flowing from you to the glass, are reflected to the other side of the room.

Charles. Then if I go to that part, I shall see the rays of light flowing from my brother :-and I do see him

in the glass.

James.

Tutor.

And I see Charles.

Now the rays of light

flow from each of you to the glass, and are reflected to one another: but neither of you sees himself.

Charles. No: I will move in front of the glass, now I see myself, but not my brother; and, I think, I understand the subject very well.

Tutor. Then explain it to me by a figure, which you may draw on the

slate.

Charles. Let A B (Plate I. Fig. 1.) represent the looking-glass: if I stand at c, the rays flow from me to the glass, and are reflected back in the

same line, because now there is no angle of incidence, and of course no angle of reflection; but if I stand at r, then the rays flow from me to the glass, but they make the angle ro c, and therefore they must be reflected in the line o y, so as to make the angle y o c, which is the angle of reflection, equal to the angle ro c. And if James stand at y, he will see me at r, and I standing at 2, shall see him at y.

CONVERSATION III.

1

Of the Refraction of Light.

CHARLES.If glass stop the rays of light, and reflect them, why cannot I see myself in the window?

Tutor. It is the silvering on the glass which causes the reflection, otherwise the rays would pass through it without being stopped, and if they were not stopped, they could not be reflected. No glass, however, is so transparent, but it reflects some rays:

put your hand to within three or four inches of the window, and you see clearly the image of it.

James. So I do, and the nearer the hand is to the glass, the more evident is the image, but it is formed on the other side of the glass, and beyond it too.

Tutor. It is; this happens, also in looking-glasses: you do not see yourself on the surface, but apparently as far behind the glass, as you stand from it in the front.

Whatever suffers the rays of light to pass through it, is called a medium. Glass, which is transparent, is a medium; so also is air, water, and indeed all fluids that are transparent are called media, and the more transVOL. V.

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