quantity of heat: hence concave mirrors have obtained the name of burning-mirrors.

Emily. I have often heard of the surprising effects of burning-mirrors, and I am quite delighted to understand their nature.

Caroline. It cannot be the true focus of the mirror at which the rays of the sun unite, for as they proceed from a point, they must fall divergent upon the mirror.

Mrs. B. Strictly speaking, they certainly do. But when rays come from such an immense distance as the sun, their divergence is so trifling, as to be imperceptible; and they may be considered as parallel: their point of union is, therefore, the true focus of the mirror, and there the image of the object is represented.

Now that I have removed the mirror out of the influence of the sun's rays, if I place a burning taper in the focus, how will its light be reflected? (Fig. 6.) Caroline. That, I confess, I cannot say.

Mrs. B. The ray which falls in the direction of the axis of the mirror, is reflected back in the same line; but let us draw two other rays from the focus, falling on the mirror at B and F; the dotted lines are perpendicular to those points, and the two rays will therefore be reflected to A and E.

Caroline. Oh, now I understand it clearly. The rays which proceed from a light placed in the focus of a concave mirror fall divergent upon it, and are reflected parallel. It is exactly the reverse of the former experiment, in which the sun's rays fell parallel on the mirror, and were reflected to a focus.

Mrs. B. Yes : when the incident rays are parallel, the reflected rays converge to a focus; when, on the contrary, the incident rays proceed from the focus, they are reflected parallel. This is an important law of optics, and since you are now acquainted with the principles on which it is founded, I hope that you will not forget it.

Caroline. I am sure that we shall not. But, Mrs. B., you said that the image was formed in the focus of

Emily. In what direction does the water attract the ray?

Mrs. B. It must attract it perpendicularly towards it, in the same manner as gravity acts on bodies.

If then a ray, A B, (fig. 1. plate XIX.) fall perpendicularly on water, the attraction of the water acts in the same direction as the course of the ray: it will not therefore cause a deviation, and the ray will proceed straight on to E. But if it fall obliquely, as the ray C B, the water will attract it out of its course. Let us suppose the ray to have approached the surface of a denser medium, and that it there begins to be affected by its attraction; this attraction, if not counteracted by some other power, would draw it perpendicularly to the water, at B; but it is also impelled by its projectile force, which the attraction of the denser medium cannot overcome; the ray, therefore, acted on by both these powers, moves in a direction between them, and instead of pursuing its original course to D, or being implicitly guided by the water to E, proceeds towards F, so that the ray appears bent or broken.

Caroline. I understand that very well; and is not this the reason that oars appear bent in water? Mrs. B. It is owing to the refraction of the rays reflected by the oar; but this is in passing from a dense to a rare medium, for you know that the rays, by means of which you see the oar, pass from water into air.

Emily. But I do not understand why a refraction takes place when a ray passes from a dense into a rare medium; I should suppose that it would be rather less, than more, attracted by the latter.

Mrs. B. And it is precisely on that account that the ray is refracted. C B, fig. 2. represents a ray passing obliquely from the glass into water: glass being the denser medium, the ray will be more strongly attracted by that which it leaves than by that which it enters. The attraction of the glass acts in the direction A B, while the impulse of projection would carry the ray to

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F; it moyes, therefore, between these directions towards D.

Emily. So that a contrary refraction takes place when a ray passes from a dense into a rare medium. Caroline. But does not the attraction of the denser medium affect the ray before it touches it?

Mrs. B. The distance at which the attraction of the denser medium acts upon a ray is so small as to be insensible; it appears therefore to be refracted only at the point at which it passes from one medium to the other.

Now that you understand the principle of refraction, I will show you the refraction of a real ray of light. Do you see the flower painted at the bottom of the inside of this tea-cup? (Fig. 3.)

Emily. Yes. But now you have moved it just out of sight, the rim of the cup hides it.

Mrs. B. Do not stir. I will fill the cup with water, and you will see the flower again.

Emily. I do indeed! Let me try to explain this: when you draw the cup from me so as to conceal the flower, the rays reflected by it no longer met my eyes, but were directed above them; but now that you have filled the cup with water, they are refracted by the attraction of the water, and bent downwards, so as again to enter my eyes.

Mrs. B. You have explained it perfectly: fig. 3. will help to imprint it on your memory. You must observe that when the flower becomes visible by the refraction of the ray, you do not see it in the situation which it really occupies, but an image of the flower higher in the cup; for as objects always appear to be situated in the direction of the rays which enter the eye, the flower will be seen in the direction of the reflected ray B. Emily. Then, when we see the bottom of a clear stream of water, the rays which it reflects being refracted in their passage from the water into the air, will make the bottom appear higher than it really is.

Mrs. B. And the water will consequently appear