Part 83

Mrs. B. Not in painting; but it may be so in the primitive rays of the spectrum. Dr. Wollaston observed, that, by increasing the breadth of the aperture, by which the line of light was admitted, the space occupied by each coloured ray in the spectrum, was augmented, in proportion as each portion encroached on the neighbouring colour, and mixed with it; so that the intervention of orange and yellow, between the red and green, is owing, he supposes, to the mixture of these two colours; and the blue is blended on the one side with the green, and on the other with the violet, forming the spectrum, as it was originally observed by Sir Isaac Newton, and which I have just shown you.

The rainbow, which exhibits a series of colours, so analogous to those of the spectrum, is formed by the refraction of the sun's rays, in their passage through a shower of rain; every drop of which acts as a prism, in separating the coloured rays as they pass through it; the combined effect of innumerable drops, produces the bow, which you know can be seen, only when there are both rain, and sunshine.

Emily. Pray, Mrs. B., cannot the sun's rays be collected to a focus by a lens, in the same manner as they are by a concave mirror?

Mrs. B. The same effect in concentrating the rays, is produced by the refraction with a lens, as by the reflection from a concave mirror: in the first, the rays pass through the glass and converge to a focus, behind it, in the latter, they are reflected from the mirror, and brought to a focus, before it. A lens, when used for the purpose of collecting the sun's rays, is called a burning glass. I have before explained to you, the manner in which a convex lens, refracts the rays, and brings them to a focus; (fig. 6, plate 19.) as these rays contain both light and heat, the latter, as well as the former, is refracted; and intense heat, as well as light, will be found in the focal point. The sun now shines very bright; if we let the rays fall on this lens, you will perceive the focus.

Emily. Oh yes: the point of union of the rays, is very luminous. I will hold a piece of paper in the focus, and see if it will take fire. The spot of light is extremely brilliant, but the paper does not burn?

Mrs. B. Try a piece of brown paper;--that, you see, takes fire almost immediately.

Caroline. This is surprising; for the light appeared to shine more intensely, on the white, than on the brown paper.

Mrs. B. The lens collects an equal number of rays to a focus, whether you hold the white or the brown paper, there; but the white paper appears more luminous in the focus, because most of the rays, instead of entering into the paper, are reflected by it; and this is the reason that the paper does not readily take fire: whilst, on the contrary, the brown paper, which absorbs more light and heat than it reflects, soon becomes heated and takes fire.

Caroline. This is extremely curious; but why should brown paper, absorb more rays, than white paper?

Mrs. B. I am far from being able to give a satisfactory answer to that question. We can form but mere conjecture on this point; it is supposed that the tendency to absorb, or reflect rays, depends on the arrangement of the minute particles of the body, and that this diversity of arrangement renders some bodies susceptible of reflecting one coloured ray, and absorbing the others; whilst other bodies, have a tendency to reflect all the colours, and others again, to absorb them all.

Emily. And how do you know which colours bodies have a tendency to reflect, or which to absorb?

Mrs. B. Because a body always appears to be of the colour which it reflects; for, as we see only by reflected rays, it can appear of the colour of those rays, only.

Caroline. But we see all bodies of their own natural colour, Mrs. B.; the grass and trees, green; the sky, blue; the flowers of various hues.

Mrs. B. True; but why is the grass green?--because it absorbs all, except the green rays; it is, therefore, these only which the grass and trees reflect to our eyes, and this makes them appear green. The flowers, in the same manner, reflect the various colours of which they appear to us; the rose, the red rays; the violet, the blue; the jonquil, the yellow, &c.

Caroline. But these are the permanent colours of the grass and flowers, whether the sun's rays shine on them or not.

Mrs. B. Whenever you see those colours, the flowers must be illumined by some light; and light, from whatever source it proceeds, is of the same nature; composed of the various coloured rays which paint the grass, the flowers, and every coloured object in nature.

Caroline. But, Mrs. B., the grass is green, and the flowers are coloured, whether in the dark, or exposed to the light?

Mrs. B. Why should you think so?

Caroline. It cannot be otherwise.

Mrs. B. A most philosophical reason indeed! But, as I never saw them in the dark, you will allow me to dissent from your opinion.

Caroline. What colour do you suppose them to be, then, in the dark?

Mrs. B. None at all; or black, which is the same thing. You can never see objects, without light. White light is compounded of rays, from which all the colours in nature are produced; there, therefore, can be no colour without light; and though a substance is black, or without colour, in the dark, it may become coloured, as soon as it becomes visible. It is visible, indeed, only by the coloured rays which it reflects; therefore, we can see it only when coloured.

Caroline. All you say seems very true, and I know not what to object to it; yet it appears at the same time incredible! What, Mrs. B., are we all as black as negroes in the dark? you make me shudder at the thought.

Mrs. B. Your vanity need not be alarmed at the idea, as you are certain of never being seen, in that state.

Caroline. That is some consolation, undoubtedly; but what a melancholy reflection it is, that all nature which appears so beautifully diversified with colours, is really one uniform mass of blackness!

Mrs. B. Is nature less pleasing for being coloured, as well as illumined, by the rays of light? and are colours less beautiful, for being accidental, rather than essential properties of bodies?

Providence seems to have decorated nature with the enchanting diversity of colours, which we so much admire, for the sole purpose of beautifying the scene, and rendering it a source of sensible gratification: it is an ornament which embellishes nature, whenever we behold her. What reason is there to regret, that she does not wear it when she is invisible?

Emily. I confess, Mrs. B., that I have had my doubts, as well as Caroline, though she has spared me the pains of expressing them: but I have just thought of an experiment, which, if it succeed, will, I am sure, satisfy us both. It is certain, that we cannot see bodies in the dark, to know whether they have then any colour. But we may place a coloured body in a ray of light, which has been refracted by a prism; and if your theory is true, the body, of whatever colour it naturally is, must appear of the colour of the ray in which it is placed; for since it receives no other coloured rays, it can reflect no others.