Part 82

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 on the refracting 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.

The property of those lenses 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 and C, falling on the concave lens X Y, (fig. 7, plate 19.) instead of converging towards the ray B, in 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 seconds, to d, e.

Caroline. And lenses which have one side flat, and the other convex, or concave, as A and B, (fig. 1, plate 20.) are, I suppose, less powerful in their refractions?

Mrs. B. Yes; the focus of the plano-convex, is at the distance of the diameter of a sphere, of which the convex surface of the lens, forms a portion; as represented in figure 2, plate 20. The three parallel rays, A B C, are brought to a focus by the plano-convex lens, X Y, at F.

Emily. You have not explained to us, Mrs. B., how the lens serves to magnify objects.

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.

Mrs. B. The white rays of the sun, are composed of rays, which, when separated, produce all these colours, although when blended together, they 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 refraction. 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 circular piece of card, in compartments, with these seven colours, as nearly as possible in the proportion, and of the shade exhibited in the spectrum, and whirl it rapidly on a pin, it will appear white; as the velocity of the motion, will have the effect of blending the colours, in the impression which they make upon the eye.

But a more decisive proof of the composition of a white ray is afforded, by reuniting these coloured 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 coloured rays, which have been separated by a prism, fall upon a lens, which will converge them to a focus; and if, when thus reunited, we find that they appear white as they did before refraction, I hope you will be convinced that the white rays, are a compound of the several coloured rays. The prism P, you see, (fig. 5.) separates a ray of white light, into seven coloured rays, and the lens L L brings them to a focus at F, where they again appear white.

Caroline. You succeed to perfection: this is indeed a most interesting and conclusive experiment.

Emily. Yet, Mrs. B., I cannot help thinking, that there may, perhaps, be but three distinct colours in the spectrum, red, yellow, and blue; and that the four others may consist of two of these colours blended together; for, in painting, we find, that by mixing red and yellow, we produce orange; with different proportions of red and blue, we make violet or any shade of purple; and yellow, and blue, form green. Now, it is very natural to suppose, that the refraction of a prism, may not be so perfect as to separate the coloured rays of light completely, and that those which are contiguous, in order of refrangibility, may encroach on each other, and by mixing, produce the intermediate colours, orange, green, violet, and indigo.

Mrs. B. Your observation is, I believe, neither quite wrong, nor quite right. Dr. Wollaston, who has performed many experiments on the refraction of light, in a more accurate manner than had been previously done, by receiving a very narrow line of light on a prism, found that it formed a spectrum, consisting of rays of four colours only; but they were not exactly those you have named as primitive colours, for they consisted of red, green, blue, and violet. A very narrow line of yellow was visible, at the limit of the red and green, which Dr. Wollaston attributed to the overlapping of the edges of the red and green light.

Caroline. But red and green mixed together, do not produce yellow?