parent 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 for mer course, and this is called refrac

tion.

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 sa, to fall upon it at a, it will not pass through in the direction 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 r, and when it comes into the air, it will pass on in the direction r z, which is parallel to ss.

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

D2

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 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 r a pass through glass or water into air, it will not, when it comes to a, move in

the direction am, but in the line a s, which is farther 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?

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 r: now when the pan is empty, the rays of light flow from r, in the direction tam, but your eye is at s, of course you cannot see any thing by the ray proceeding along x a m. I put the water into the

As soon as

vessel, the rays of light proceed from 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.