_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? [Illustration: PLATE XXI.] _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. _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?