Part 84

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 passing 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 quantity 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 passing 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 passing through the other colours of the spectrum.

Caroline. They must, however, reflect great quantities 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 quantity 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 passing 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?

Mrs. B. It is supposed to be owing to the rays, which are most refrangible, being also the most easily reflected: in passing 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, pass 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 passage to the earth.

Mrs. B. Do not forget that the direct rays of light which pass 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 pass 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.