Mrs. B. By turning again to [fig. 6, plate 19.] you will readily understand this. Let A C, be an object placed before the lens, and suppose it to be seen by an eye at F; the ray from the point A, will be seen in the direction F G, that from C, in the direction F H; the visual angle, therefore, will be greatly increased, and the object must appear larger, in proportion.
I must now explain to you the refraction of a ray of light, by 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 suppose 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 by a prism, can produce.
Mrs. B. The refractions of the ray, both on entering and on quitting the prism, are 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. In the first instance it is refracted towards, and in the last, from the perpendicular; each causing it to deviate in the same way, from its original course, A B.
I will show you this by experiment; 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 colours are represented on the opposite wall! There are all the colours of the rainbow, and with a brightness, I never saw equalled. ([Fig. 4, plate 20.])
Emily. I have seen an effect, in some respects 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 colours?
Mrs. B. The colours are not formed by the prism, but existed in the ray previously to its refraction.
Caroline. Yet, before its refraction, it appeared perfectly white.