Mrs. B. The focal distance depends both upon the form of the lens, and of the refractive power of the substance of which it is made in a glass lens, both sides of which are equally convex, the focus is situated nearly at the centre of the sphere of which the surface of the lens forms a portion; it is at the distance, therefore, of the radius of the sphere.

There are lenses of various forms, as you will find described in fig. 1. plate XX. The property of those which have a convex surface is to collect the rays of light to a focus; and of those which have a concave surface, on the contrary, to disperse them. For the rays A C falling on the concave lens X Y, (fig. 7. plate XIX.) instead of converging towards the ray B, which falls on the axis of the lens, will each be attracted towards the thick edges of the lens, both on entering and quitting it, and will, therefore, by the first refraction, be made to diverge to a, c, and by the second to d, e.

Caroline. And lenses which have one side flat and the other convex or concave, as A and B, fig. 1. plate XX., are, I suppose, less powerful in their refractions P

Mrs. B. Yes; they are called plano-convex, and plano-concave lenses: the focus of the former is at the distance of the diameter of a sphere, of which the convex surface of the lens forms a portion; as represented in fig. 2. plate XX. The three parallel rays, A B C, are brought to a focus by the plano-convex lens, X Y at F.

I must now explain to you the refraction of a triangular piece of glass, called a prism. (Fig. 3.)

Emily. The three sides of this glass are flat; it cannot therefore bring the rays to a focus; nor do I sup

A

pose that its refraction will be similar to that of a flat pane of glass, because it has not two sides parallel; I cannot therefore conjecture what effect the refraction of a prism can produce.

Mrs. B. The refractions of the light, on entering and on quitting the prism, are both in the same direction. (Fig. 3.) On entering the prism P, the ray A is refracted from B to C, and on quitting it from C to D.

I will show you this in nature; but for this purpose it will be advisable to close the window-shutters, and admit, through the small aperture, a ray of light, which I shall refract by means of this prism.

Caroline. Oh, what beautiful colors are represented on the opposite wall! There are all the colors of the rainbow, and with a brightness I never saw equalled. (Fig. 4. plate XX.)

Emily. I have seen an effect, in some respect similar to this, produced by the rays of the sun shining upon glass lustres; but how is it possible that a piece of white glass can produce such a variety of brilliant colors?

Mrs. B. The colors are not formed by the prism, but existed in the ray previous to its refraction.

Caroline. Yet, before its refraction, it appeared perfectly white.

Mrs. B. The white rays of the sun are composed of colored rays, which, when blended together, appear colorless or white.

Sir Isaac Newton, to whom we are indebted for the most important discoverics respecting light and colors, was the first who divided a white ray of light, and found it to consist of an assemblage of colored rays, which

formed an image upon the wall, such as you now see exhibited, (fig. 4.) in which are displayed the following series of colors: red, orange, yellow, green, blue, indigo, and violet.

Emily. But how does a prism separate these colored rays?

Mrs. B. By refraction. It appears that the colored rays have different degrees of refrangibility; in passing through the prism, therefore, they take different directions according to their susceptibility of refraction. The violet rays deviate most from their original course; they appear at one of the ends of the spectrum AB: contiguous to the violet, are the blue rays, being those which have somewhat less refrangibility; then follow, in succession, the green, yellow, orange, and, lastly, the red, which are the least refrangible of the colored rays.

Caroline, I cannot conceive how these colors, mixed together, can become white?

Mrs. B. That I cannot pretend to explain; but it is a fact that the union of these colors, in the proportions in which they appear in the spectrum, produce in us the idea of whiteness. If you paint a card in compartments with these seven colors, and whirl it rapidly on a pin, it will appear white.

But a more decisive proof of the composition of a white ray is afforded by reuniting these colored rays, and forming with them a ray of white light.

Caroline. If you can take a ray of white light to pieces, and put it together again, I shall be quite satisfied.

Mrs. B This can be done by letting the colored