Caroline. I am sure that we shall not. But, Mrs. B., you said that the image was formed in the focus of a concave mirror; yet I have frequently seen glass concave mirrors, where the object has been represented within the mirror, in the same manner as in a convex mirror.

Mrs. B. That is the case only, when the object is placed between the mirror and its focus; the image then appears magnified behind, or, as you call it, within the mirror.

Caroline. I do not understand why the image should be larger than the object.

Mrs. B. It proceeds from the convergent property of the concave mirror. If an object, A B, (fig. 7.) be placed between the mirror and its focus, the rays from its extremities fall divergent on the mirror, and on being reflected, becomes less divergent, as if they proeeeded from C: to an eye placed in that situation the image will appear magnified behind the mirror at a b, since it is seen under a larger angle than the object.

You now, I hope, understand the reflection of light by opaque bodies. At our next meeting, we shall enter upon another property of light no less interesting which is called refraction.

CONVERSATION XVI.

ON REFRACTION AND COLORS.

Transmission of Light by Transparent Bodies.— Refraction.-Refraction of the Atmosphere.-Refraction of a Lens.-Refraction of the Prism.-Of the Colors of Rays of Light.-Of the Colors of Bodies.

MRS. B.

THE refraction of light will furnish the subject of to-day's lesson.

Caroline. That is a property of which I have not the faintest idea.

Mrs. B. It is the effect which transparent mediums produce on light in its passage through them. Opaque bodies, you know, reflect the rays,and transparent bodies transmit them; but it is found, that if a ray, in passing from one medium into another of different density, fall obliquely, it is turned out of its course.

Caroline. It must then be acted on by some new power, otherwise it would not deviate from its first direction.

Mrs. B. The power which causes the deviation of the ray appears to be the attraction of the denser medium. Let us suppose the two mediums to be air and water; if a ray of light passes from air into water it is more strongly attracted by the latter on account of its superior density.

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