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.