SKIN-DRESSING. [LEATHER MANUFACTURE.] SKY is the name commonly applied to the infinite space which surrounds the earth, and of which the visible portion, above the horizon of a spectator on any part of the earth's surface, appears to have the form of a concave segment less than a hemisphere.
The earth is surrounded by the atmosphere, which is charged with vapours and terrestrial particles; and by the reflections which, in con sequence, the rays of light experience in passing to the spectator, and the absorption which they undergo in their long transit of the atmo sphere when the sun is near the horizon, the sky assumes the variously coloured tints under which it is seen.
It is known from many experiments that pure air is devoid of colour; and the observations of M. de Saussure (` Voyages dans les Alpes,' tom. 4, p. 239) have established the fact, that, in an atmosphere free from vapours, such objects as mountains covered with snow, when seen by a spectator at a distance of 20 or 30 leagues from them, by light which is merely transmitted through the air, appear to be white : the same philosopher has observed, however, that at times when the sun is seen to set behind a mountain so covered, the blue rays reflected from the sky to the mountain, and from thence to the spectator, cause the mountain to assume a blue colour, the other light with which the mountain is illuminated not being strong enough to overpower the blue reflected rays.
When the sun has considerable elevation, the rays of light which pass through the earth's atmosphere almost perpendicularly to its surface, undergo scarcely any change of direction ; hut, with respect to the light from the sun which enters obliquely into the atmosphere, the violet and blue rays are partially arrested in their course, and are re fleeted in abundance to the earth ; they thus, when the atmosphere is nearly free from clouds, give to the parts of the sky which are remote from the apparent place of the sun an azure tint.
The blue colour of the sky about the zenith increases in proportion as the sun is nearer the horizon; and, at the same time, the blue rays in the beams of light which traverse the atmosphere in directions nearly parallel to the horizon are absorbed, so that it is chiefly those of less refrangibility, such as the yellow and the red, that arrive at the eye of the epectator : in consequence of this the sky near the horizon, on the side which is towards the sun, appears to be highly tinted with those colours.
Many of the blue rays, after reflection from the upper parts of the atmosphere, are, however, absorbed in passing down to the earth ; and iurther, it seems probable that iu that portion of the reflection which takes place at a great elevation the blue rays are reflected in larger proportion compared with other colours than lower down; and hence it is that the blue tint of the sky is found to increase in intensity as the spectator ascends above the general surface of the • — — earth. On the top of high mountains, or in balloons at great eleva tions, the proportion of blue rays which, after reflection in the atmo sphere enter the eye is very great; and the blueness at length becomes a deep black ground on which the stars appear to shine at all times with as much lustre as at midnight on the earth, during the absence of the moon. It is hence evident that if it were not for the innumerable
reflections of the light from the sun or moon, which take place in the atmosphere, total darkness would prevail from the instant of sunset to that of inn-rise ; and even during the day, darkness would creme, so that the stars would become visible, every time that the sun is obscured by a cloud of sufficient density.
That the blue tint of the sky is caused by light reflected in the atmosphere, is abundantly evident from the fact that the light of the sky is found to be polarized, this quality in light being a result of its reflection. Sir David Brewster, who first made the observation, has moreover ascertained that the light of the sky consists of two parts, one blue and the other nearly without colour ; and he discovered that these lights are polarized in different directions. (` Treatise on New Philosophical p. 349.) The hypothesis that the azure colour of the sky is caused by re flections of blue rays, was at one time objected to on the ground that the shadows of opaque objects, placed on white paper and exposed to the sun's light, should always appear to be blue, since the part of the paper which is in shadow can only be visible by the light of the sky reflected from thence. This phenomenon is, in fact, frequently observed; but M. do Saussure, while admitting that he has often per ceived the shadows of objects to be bluish in the mornings and ' evenings on the general surface of the earth, states that in Alpine regions, where the sky is intensely blue, the shadows of objects never appear to be so : he adds that, of fifty-nine observations made for the purpose of ascertaining the colours of shadows on the mountains, thirty-four showed them to be a pale at let, eighteen showed them to be black, six a pale blue, and once they appeared to be yellowish. It may be inferred, therefore, that shadows cast by opaque objects are so much affected by the colours of the neighbouring objects that a right judgment can scarcely be formed of the colour which they receive from the light of the sky. [ACCIDENTAL COLOURS.] To the like interference must be ascribed the variously-coloured shadows which were observed by 11. Botiguer ('Essai d'Optique ;' and Id. Buffon, • 31dmoiren de l'Araddmie des Sciences,' 1743.) Between the tropics the transparency of the atmosphere is far gretter than it is, in general, in regions beyond them towards the north or south ; hence the sky there is almost always serene and intensely blue, while the clouds near the setting sun are brightly tinted with the prismatic colours. The skies of the south of Europe and some parts of North America are distinguished for their serenity and beauty ; hut, in these respects, they arc said to be inferior to the skies over the islands in the Pacific Ocean.
For the description of an instrument invented in order to measure the intensity of the blue colour in the sky, see CYANOMETER.