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 CB, 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.

ray

Mrs. B. And it is precisely on that account that the 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

passes through the pane to D; at that point returning into the air it is again refracted by the glass, but in a contrary direction to the first refraction, and in conse quence proceeds to E. Now you must observe that the ray B C and the ray D E being parallel, the light does not appear to have suffered any refraction.

Emily. So that the effect which takes place on the ray entering the glass, is undone on its quitting it. Or, to express myself more scientifically, when a ray of light passes from one medium into another, and through that into the first again, the two refractions being equal and in opposite directions, no sensible effect is produced.

Mrs. B. This is the case when the two surfaces of the refracting medium are parallel to each other; if they are not, the two refractions may be made in the same direction, as I shall show you.

When parallel rays (fig. 6.) fall on a piece of glass having a double convex surface, and which is called a Lens, that only which falls in the direction of the axis of the lens is perpendicular to the surface; the other rays falling obliquely are refracted towards the axis, and will meet at a point beyond the lens, called its focus.

Of the three rays, A B C, which fall on the lens D E, the rays A and C are refracted in their passage through it, to a, and c, and on quitting the lens they undergo a second refraction in the same direction which unites them with the ray B, at the focus F.

Emily. And what is the distance of the focus from the surface of the lens?

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 described in fig. 1. plate XX.

forms, as you will find 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 AC 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 reiractions?

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 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 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 shew 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 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 XX.)

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 previous to its refraction.

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

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

Sir Isaac Newton, to whom we are indebted for the most important discoveries respecting light and colours, was the first who divided a white ray of light, and found it to consist of an assemblage of coloured rays, which formed an image upon the wall, such as you now see exhibited, (fig. 4.) in which are displayed the following series of colours: red, orange, yellow, green, blue, indigo, and violet.

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

Mrs. B. By reflection. It appears that the coloured 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 A B: 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 coloured

rays.

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

Mrs. B. That I cannot pretend to explain; but it is a fact that the union of these colours, in the proportions in which they appear in the spectrum, produce in us the idea of whiteness. If you paint a card in compart