Conversations on Natural Philosophy, in which the Elements of that Science are Familiarly Explained Part 47

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

_Mrs. B._ True; but why is the gra.s.s green?--because it absorbs all, except the green rays; it is, therefore, these only which the gra.s.s 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 gra.s.s and flowers, whether the sun's rays s.h.i.+ne 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 gra.s.s, the flowers, and every coloured object in nature.

_Caroline._ But, Mrs. B., the gra.s.s 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 ma.s.s 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.

_Caroline._ Oh! that is an excellent thought, Emily; will you stand the test, Mrs. B.?

_Mrs. B._ I consent: but we must darken the room, and admit only the ray which is to be refracted; otherwise, the white rays will be reflected on the body under trial, from various parts of the room. With what do you choose to make the experiment?

_Caroline._ This rose: look at it, Mrs. B., and tell me whether it is possible to deprive it of its beautiful colour?

_Mrs. B._ We shall see.--I expose it first to the red rays, and the flower appears of a more brilliant hue; but observe the green leaves----

_Caroline._ They appear neither red nor green; but of a dingy brown with a reddish glow?

_Mrs. B._ They cannot appear green, because they have no green rays to reflect; neither are they red, because green bodies absorb most of the red rays. But though bodies, from the arrangement of their particles, have a tendency to absorb some rays, and reflect others, yet it is not natural to suppose, that bodies are so perfectly uniform in their arrangement, as to reflect only pure rays of one colour, and perfectly to absorb the others; it is found, on the contrary, that a body reflects, in great abundance, the rays which determine its colour, and the others in a greater or less degree, in proportion as they are nearer to or further from its own colour, in the order of refrangibility. The green leaves of the rose, therefore, will reflect a few of the red rays, which, blended with their natural blackness, give them that brown tinge: if they reflected none of the red rays, they would appear perfectly black. Now I shall hold the rose in the blue rays----

_Caroline._ Oh, Emily, Mrs. B. is right! look at the rose: it is no longer red, but of a dingy blue colour.

_Emily._ This is the most wonderful, of any thing we have yet learnt.

But, Mrs. B., what is the reason that the green leaves, are of a brighter blue than the rose?

_Mrs. B._ The green leaves reflect both blue and yellow rays, which produce a green colour. They are now in a coloured ray, which they have a tendency to reflect; they, therefore, reflect more of the blue rays than the rose, (which naturally absorbs that colour,) and will, of course, appear of a brighter blue.

_Emily._ Yet, in pa.s.sing the rose through the different colours of the spectrum, the flower takes them more readily than the leaves.

_Mrs. B._ Because the flower is of a paler hue. Bodies which reflect all the rays, are white; those which absorb them all, are black: between these extremes, bodies appear lighter or darker, in proportion to the quant.i.ty of rays they reflect or absorb. This rose is of a pale red; it approaches nearer to white than to black, and therefore, reflects rays, more abundantly than it absorbs them.

_Emily._ But if a rose has so strong a tendency to reflect rays, I should imagine that it would be of a deep red colour.

_Mrs. B._ I mean to say, that it has a general tendency to reflect rays.

Pale coloured bodies, reflect all the coloured rays to a certain degree, their paleness, being an approach towards whiteness: but they reflect one colour more than the rest: this predominates over the white, and determines the colour of the body. Since, then, bodies of a pale colour, in some degree reflect all the rays of light, in pa.s.sing through the various colours of the spectrum, they will reflect them all, with tolerable brilliancy; but will appear most vivid, in the ray of their natural colour. The green leaves, on the contrary, are of a dark colour, bearing a stronger resemblance to black, than to white; they have, therefore, a greater tendency to absorb, than to reflect rays; and reflecting very few of any, but the blue, and yellow rays, they will appear dingy, in pa.s.sing through the other colours of the spectrum.

_Caroline._ They must, however, reflect great quant.i.ties of the green rays, to produce so deep a colour.

_Mrs. B._ Deepness or darkness of colour, proceeds rather from a deficiency, than an abundance of reflected rays. Remember, that if bodies reflected none of the rays, they would be black; and if a body reflects only a few green rays, it will appear of a dark green; it is the brightness, and intensity of the colour, which show that a great quant.i.ty of rays are reflected.

_Emily._ A white body, then, which reflects all the rays, will appear equally bright in all the colours of the spectrum.

_Mrs. B._ Certainly. And this is easily proved by pa.s.sing a sheet of white paper, through the rays of the spectrum.

White, you perceive, results from a body reflecting all the rays which fall upon it; black, is produced, when they are all absorbed; and colour, arises from a body possessing the power to decompose the solar ray, by absorbing some parts, and reflecting others.

_Caroline._ What is the reason that articles which are blue, often appear green, by candle-light?

_Mrs. B._ The light of a candle, is not of so pure a white as that of the sun: it has a yellowish tinge, and when refracted by the prism, the yellow rays predominate; and blue bodies reflect some of the yellow rays, from their being next to the blue, in the order of refrangibility; the superabundance of yellow rays, which is supplied by the candle, gives to blue bodies, a greenish hue.

_Caroline._ Candle-light must then give to all bodies, a yellowish tinge, from the excess of yellow rays; and yet it is a common remark, that people of a sallow complexion, appear fairer, or whiter, by candle-light.

_Mrs. B._ The yellow cast of their complexion is not so striking, when every surrounding object has a yellow tinge.

_Emily._ Pray, why does the sun appear red, through a fog?

[Ill.u.s.tration: PLATE XXI.]

_Mrs. B._ It is supposed to be owing to the rays, which are most refrangible, being also the most easily reflected: in pa.s.sing through an atmosphere, loaded with moisture, as in foggy weather, and also in the morning and evening, when mists prevail, the _violet_, _indigo_, _blue_, and _green_ rays, are reflected back by the particles which load the air; whilst the _yellow_, _orange_, and _red_ rays, being less susceptible of reflection, pa.s.s on, and reach the eye.

_Caroline._ And, pray, why is the sky of a blue colour?

_Mrs. B._ You should rather say, the atmosphere; for the sky is a very vague term, the meaning of which, it would be difficult to define, philosophically.

_Caroline._ But the colour of the atmosphere should be white, since all the rays traverse it, in their pa.s.sage to the earth.

_Mrs. B._ Do not forget that the direct rays of light which pa.s.s from the sun to the earth, do not meet our eyes, excepting when we are looking at that luminary, and thus intercept them; in which case, you know, that the sun appears white. The atmosphere is a transparent medium, through which the sun's rays pa.s.s freely to the earth; but the particles of which it is composed, also reflect the rays of light, and it appears that they possess the property of reflecting the blue rays, the most copiously: the light, therefore, which is reflected back into the atmosphere, from the surface of the earth, falls upon these particles of air, and the blue rays are returned by reflection: this reflection is performed in every possible direction; so that whenever we look at the atmosphere, some of these rays fall upon our eyes; hence we see the air of a blue colour. If the atmosphere did not reflect any rays, though the objects, on the surface of the earth, would be illuminated, the sky would appear perfectly black.

_Caroline._ Oh, how melancholy would that be; and how pernicious to the sight, to be constantly viewing bright objects against a black sky. But what is the reason that bodies often change their colour; as leaves, which wither in autumn, or a spot of ink, which produces an iron-mould on linen?

_Mrs. B._ It arises from some chemical change, which takes place in the arrangement of the component parts; by which they lose their tendency to reflect certain colours, and acquire the power of reflecting others. A withered leaf thus no linger reflects the blue rays; it appears, therefore, yellow, or has a slight tendency to reflect several rays, which produce a dingy brown colour.

An ink spot on linen, at first absorbs all the rays; but, from the action of soap, or of some other agent, it undergoes a chemical change, and the spot partially regains its tendency to reflect colours, but with a preference to reflect the yellow rays, and such is the colour of the iron-mould.

_Emily._ Bodies, then, far from being of the colour which they appear to possess, are of that colour to which they have the greatest aversion, with which they will not incorporate, but reject, and drive from them.