The opacity of the atmosphere, as a white film over the darkness of space, occasions the blue appearance of the sky; and in proportion to the rarity or density of the medium, is the intensity of colour, or rather depth of tone. If the atmosphere be extremely rare, as in the Polar regions, or at the height of Mont Blanc, the sky appears almost black. And if the atmosphere be thick with vapour, the sky assumes a milky colour, and the blue tint is lost in that of the medium. When the atmosphere is just so rare as to be scarcely perceptible in its influence upon terrestrial objects,—as in Italy, or the eastern climes, where the most distant buildings appear diminished in size, but almost as distinct as those close to the spectator,—yet sufficiently dense to become a veil to the expanse of space, the colour of the sky appears the most intense blue. As near as we can superficially ascertain it,—in the exact medium between such rarity of atmosphere as would afford blackness, and such opacity as would afford whiteness,—we may expect to find the most intensely blue colour in the sky.
As the effect of this colourless opacity of the atmosphere is, to render the appearance of the darkness of space a blue colour, so all dark terrestrial objects are similarly affected by the intervening of this medium, and, in a corresponding degree, become more or less blue. The dark mountains in Wales and Scotland appear of a deep blue, sometimes verging upon purple; and a slight comparison between the colour of the trees close to the spectator and those in the distance, will show how much more blue the latter become, from the influence of the medium through which they are viewed.
And as objects, in proportion to their distance, are more or less affected by the interposition of the atmosphere, so, also, do the parts of the individual objects themselves, become more or less grey as they recede. The boundaries of a white object are less white, and of a black object less black, than the parts nearest the eye. A tree is most green at the prominent parts, and greyer at the top and sides.
This truth is so decidedly felt by the public in general—though perhaps insensibly appreciated and but tacitly acknowledged,—that, as the atmosphere reduces the colours of all objects to a blue tint, so all blue colours convey an impression of distance, and all tints approaching to blue are accordingly designated retiring colours.
But the atmosphere is not always colourless. The rays of the sun tinge it with yellow. The rays from a fire or candle tinge it with a colour approaching to red. The combination of smoke tinges it with black or brown; and fogs infuse various degrees of dingy yellow. All these variations affect the colours of the objects seen through the atmosphere, and modify the degree of blue, or quality of grey, tint communicated thereby.
When the atmosphere is coloured by the light of the sun, the blue is modified, more or less, into a warm grey. But owing to the brilliancy resulting from the blaze of light, the tints remain of the utmost purity. All tendency to green is kept in subordination by the pearly tints of those parts which are in shadow. The atmosphere is rendered more dense at the same time that it is coloured by the light of the sun; but the light parts of the objects seen through it are rendered, by the same cause, so much more brilliant, that the density of the medium is partially compensated; while its full effect is apparent upon the shadows seen through it, over which a bright haze diffuses a beautiful blue tint, slightly warmed by the golden colour of the illuminating power. The contrast of the yellow tinge in the lights makes these shadows appear to incline to purple; and at sunset and sunrise, when by the greater quantity of the medium, rendered more dense by the aqueous vapours close to the earth, the colour of the sun's light is enriched to a deep golden hue approaching orange and red, the shadows assume a decidedly purple tint, of which the blue is supplied by the density, and the red by the colour, of the medium. As the light of the sun decreases, the colour of the atmosphere is more evidently tinged with red, until the sun has sunk so far below the horizon, that the shadows of night incorporating with the colour of the vapours, render them a dull grey, sometimes approaching a brown.
In proportion as the atmosphere is illumined does it also become opaque. The sky close to the sun appears much less blue than on the opposite side of the heavens. The beams of the sun, or moon, or even the rays of a candle, become so opaque, as absolutely to conceal all objects behind them.
In a glowing sunshine, the particles of the atmosphere loaded with light, produce that soft haze or caligine, "as the Italian hath it," by which the colours of every object seen through it, are assimilated in one broad, warm, grey tone, however varied the tints of the objects in reality may be.
Another singular appearance takes place in remote objects, of which no one has so fully availed himself as Turner, for the production of pictorial beauties, and the brilliancy of sunshine. The atmosphere, which becomes most visible when before shadows, is frequently so much illuminated by the sun's rays, as to make the shadows appear nearly equally light with the illuminated parts of the objects; and the only distinction between the lights and shadows is to be found in the difference of tint—the shadows being blue or purple, and the lights a warm yellow, or fleshy colour.
The practice in art, both in Oil and in Water colours, has been an imitation of the process of nature, and with similar results. It is usual in Oil to paint the distance stronger in colour than it is intended to remain, and when dry, to pass some very thin opaque colour (technically to scumble) over the whole. Thus the most perfectly aerial tints are produced. In Water Colours, owing to the different quality of the materials employed, another method is adopted. White, or any opaque pigment (except when used in conjunction with Oil painting), has a disagreeable effect; so it is considered advisable partially to wash out the too highly coloured distance, and aerial tints similar to those produced by the scumble are obtained.