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. And I see Charles.

Tutor.

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 fi gure, which you may draw on the slate.

Charles. Let A B (Plate 1. 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 x, then the rays flow from me C

VOL. III.

to the glass, but they make the angle x o c, and therefore they must be reflected in the line o y, so as to make the angle yo c, which is the angle of reflection, equal to the angle xo c. And if James stand at y, he will see me at x, and I standing at x, shall see him at y.

CONVERSATION III.

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 im age of it.

James. So I do, and the nearer the hand is to the glass, the more evident is the im

age, 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 inedium; so also is air, water, and indeed all fluids that are transparent are called media, and the more transparent the body, the more perfect is the medium.

Charles. Do the rays of light pass through these in a straight line?

Tutor. They do: but not in precisely the same direction in which they were moving before they entered it. They are bent out of their former course, and this is called refraction.

James. Can you explain this term more clearly?

Tutor. Suppose A B (Plate 1. Fig. 2.) to be a piece of glass, two or three inches thick; and a ray of light s a, to fall upon it

at a, it will not pass through in the direc tion s s, but when it comes to a, it will be bent towards the perpendicular a b, and go through the glass in the course a x, and when it comes into the air, it will pass on in the direction x z, which is parallel to s s Charles. Does this happen if the ray fall perpendicularly on the glass at p a?.

Tutor. In that case there is no refraction, but the ray proceeds in its passage through the glass, precisely in the same direction as it did before it entered it, namely, in the direction p b.

James. Refraction then takes place only when the rays fall obliquely or slantwise on the medium?

Tutor. Just so: rays of light may pass out of a rarer into a denser medium, as from air into water or glass or they may pass from a denser medium into a rarer, as from water into air.

Charles. Are the effects the same in both cases?

Tutor. They are not: and I wish you to remember the difference. When light