The doctrine we are now illustrating is one which a great many persons, especially among the fair sex, find it difficult to admit. They cannot conceive it possible that there is no colour really inherent in their splendid attire, and no tints of beauty in their countenances. ‘What,’ said a certain lady, ‘are there no colours in my shawl, and in the ribbons that adorn my head-dress—and, are we all as black as negroes in the dark; I should almost shudder to think of it.’ Such persons, however, need be in no alarm at the idea; but may console themselves with the reflection, that, when they are stripped of all their coloured ornaments in the dark, they are certain that they will never be seen by any one in that state; and therefore, there is no reason to regret the temporary loss of those beauties which light creates—when they themselves and all surrounding objects are invisible. But, to give a still more palpable proof of this position, the following popular experiments may be stated.
Take a pint of common spirit, and pour it into a soup dish, and then set it on fire; as it begins to blaze, throw a handful of salt into the burning spirit, and keep stirring it with a spoon. Several handfuls may thus be successively thrown in, and then the spectators, standing around the flame, will see each other frightfully changed, their colours being altered into a ghastly blackness, in consequence of the nature of the light which falls upon them—which produces colours very different from those of the solar light. The following experiment, as described by Sir D. Brewster, illustrates the same principle. ‘Having obtained the means of illuminating any apartment with yellow light, let the exhibition be made in a room with furniture of various bright colours, and with oil or water coloured paintings on the wall. The party which is to witness the experiment should be dressed in a diversity of the gayest colours; and the brightest coloured flowers, and highly coloured drawings should be placed on the tables. The room being at first lighted with ordinary lights, the bright and gay colours of every thing that it contains will be finely displayed. If the white lights are now suddenly extinguished, and the yellow lamps lighted, the most appalling metamorphosis will be exhibited. The astonished individuals will no longer be able to recognise each other. All the furniture of the room, and all the objects it contains, will exhibit only one colour. The flowers will lose their hues; the paintings and drawings will appear as if they were executed in China ink, and the gayest dresses, the brightest scarlets, the purest lilacs, the richest blues and the most vivid greens, will all be converted into one monotonous yellow. The complexions of the parties, too, will suffer a corresponding change. One pallid deathlike yellow, will envelope the young and the old, and the sallow face will alone escape from the metamorphosis. Each individual derives merriment from the cadaverous appearance of his neighbour, without being sensible that he is one of the ghastly assemblage.’
——Like the unnatural hue
Which autumn paints upon the perished leaf,
From such experiments as these we might conclude, that were the solar rays of a very different description from what they are now found to be, the colours which embellish the face of nature, and the whole scene of our sublunary creation would assume a new aspect, and appear very different from what we now behold around us in every landscape. We find that the stars display great diversity of colour; which is doubtless owing to the different kinds of light which are emitted from those bodies; and hence we may conclude, that the colouring thrown upon the various objects of the universe is different in every different system, and that thus, along with other arrangements, an infinite variety of colouring and of scenery is distributed throughout the immensity of creation.
The atmosphere, in consequence of its different refractive and reflective powers, is the source of a variety of colours which frequently embellish and diversify the aspect of our sky. The air reflects the blue rays most plentifully, and must therefore transmit the red, orange, and yellow, more copiously than the other rays. When the sun and other heavenly bodies are at a high elevation, their light is transmitted without any perceptible change, but when they are near the horizon, their light must pass through a long and dense track of air, and must therefore be considerably modified before it reach the eye of the observer. The momentum of the red rays being greater than that of the violet, will force their way through the resisting medium, while the violet rays will be either reflected or absorbed. If the light of the setting sun, by thus passing through a long track of air, be divested of the green, blue, indigo, and violet rays, the remaining rays which are transmitted through the atmosphere, will illuminate the western clouds, first with an orange colour; and then, as the sun gradually sinks into the horizon, the track through which the rays must pass becoming longer, the yellow and orange are reflected, and the clouds grow more deeply red, till at length the disappearance of the sun leaves them of a leaden hue by the reflection of the blue light through the air. Similar changes of colour are sometimes seen on the eastern and western fronts of white buildings. St. Paul’s Church, in London, is frequently seen at sun-set, tinged with a very considerable degree of redness; and the same cause occasions the moon to assume a ruddy colour, by the light transmitted through the atmosphere. From such atmospherical refractions and reflections are produced those rich and beautiful hues with which our sky is gilded by the setting sun, and the glowing red which tinges the morning and evening clouds, till their ruddy glare is tempered by the purple of twilight, and the reflected azure of the sky.
When a direct spectrum is thrown on colours darker than itself, it mixes with them: as the yellow spectrum of the setting sun, thrown on the green grass, becomes a greener yellow. But when a direct spectrum is thrown on colours brighter than itself, it becomes instantly changed into the reverse spectrum, which mixes with those brighter colours. Thus the yellow spectrum of the setting sun thrown on the luminous sky, becomes blue, and changes with the colour or brightness of the clouds on which it appears. The red part of light being capable of struggling through thick and resisting mediums which intercept all other colours—is likewise the cause why the sun appears red when seen through a fog,—why distant light, though transmitted through blue or green glass, appears red—why lamps at a distance, seen through the smoke of a long street, are red, while those that are near, are white. To the same cause it is owing that a diver at the bottom of the sea is surrounded with the red light which has pierced through the superincumbent fluid, and that the blue rays are reflected from the surface of the ocean. Hence, Dr. Halley informs us that, when he was in a diving bell, at the bottom of the sea, his hand always appeared red in the water.
The blue rays, as already noticed, being unable to resist the obstructions they meet with in their course through the atmosphere, are either reflected or absorbed in their passage. It is to this cause, that most philosophers ascribe the blue colour of the sky, the faintness and obscurity of distant objects, and the bright azure which tinges the mountains of a distant landscape.
SECT. 3.—PHENOMENA OF THE RAINBOW.
Since the rays of light are found to be decomposed by refracting surfaces, and reflected in an infinite variety of modes and shades of colour, we need not be surprised at the changes produced in any scene or object by the intervention of another, and by the numerous modifications of which the primary colours of nature are susceptible. The vivid colours which gild the rising and the setting sun, must necessarily differ from those which adorn its noon-day splendour. Variety of atmospheric scenery will thus necessarily be produced, greater than the most lively fancy can well imagine. The clouds will sometimes assume the most fantastic forms, and at other times will be irradiated with beams of light, or, covered with the darkest hues, will assume a lowering aspect, prognostive of the thunder’s roar and the lightning’s flash—all in accordance with the different rays that are reflected to our eyes, or the quantity absorbed by the vapours which float in the atmosphere.
Light, which embellishes with so much magnificence a pure and serene sky, by means of innumerable bright starry orbs which are spread over it, sometimes, in a dark and cloudy sky, exhibits an ornament which, by its pomp, splendour and variety of colours, attracts the attention of every eye that has an opportunity of beholding it. At certain times, when there is a shower either around us, or at a distance from us in an opposite quarter to that of the sun, a species of arch or bow is seen in the sky, adorned with all the seven primary colours of light. This phenomenon, which is one of the most beautiful meteors in nature, has obtained the name of the Rainbow. The rainbow was, for ages, considered as an inexplicable mystery, and by some nations it was adored as a deity. Even after the dawn of true philosophy, it was a considerable time before any discovery of importance was made, as to the true causes which operate in the production of this phenomenon. About the year 1571, M. Fletcher of Breslau, made a certain approximation to the discovery of the true cause, by endeavouring to account for the colours of the rainbow by means of a double refraction and one reflection. A nearer approximation was made by Antonio de Dominis, bishop of Spalatro, about 1601. He maintained that the double refraction of Fletcher, with an intervening reflection, was sufficient to produce the colours of the bow, and also to bring the rays that formed them to the eye of the spectator, without any subsequent reflection. To verify this hypothesis, he procured a small globe of solid glass, and viewing it when it was exposed to the rays of the sun—with his back to that luminary—in the same manner as he had supposed the drops of rain were situated with respect to them, he observed the same colours which he had seen in the rainbow, and in the same order. But he could give no good reason why the bow should be coloured, and much less any satisfactory account of the order in which the colours appear. It was not till Sir I. Newton discovered the different refrangibility of the rays of light, that a complete and satisfactory explanation could be given of all the circumstances connected with this phenomenon.