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a much greater portion is absorbed by the body. Bodies that reflect all the rays appear white; those that absorb them all seem black; but most substances, after decomposing the white light which falls upon them, reflect some colours and absorb the rest. A violet reflects the violet rays alone, and absorbs the others; scarlet cloth absorbs almost all the colours except red; yellow cloth reflects the yellow rays most abundantly, and blue cloth those that are blue; consequently colour is not a property of matter, but arises from the action of matter upon light. Thus a white ribbon reflects all the rays, but when dyed red, the particles of the silk acquire the property of reflecting the red rays most abundantly and of absorbing the others. Upon this property of unequal absorption, the colours of transparent media depend; for they also receive their colour from their power of stopping or absorbing some of the colours of white light and transmitting others; as, for example, black and red ink, though equally homogeneous, absorb different kinds of rays; and when exposed to the sun, they become heated in different degrees, while pure water seems to transmit all rays equally, and is not sensibly heated by the passing light of the sun. The rich dark light transmitted by a smalt-blue finger-glass is not a homogeneous colour, like the blue or indigo of the

spectrum, but is a mixture of all the colours of white light which the glass has not absorbed; and the colours absorbed are such as, mixed with the blue tint, would form white light. When the spectrum of seven colours is viewed through a thin plate of this glass, they are all visible; and when the plate is very thick, every colour is absorbed between the extreme red and the extreme violet, the interval being perfectly black. But if the spectrum be viewed through a certain thickness of the glass intermediate between the two, it will be found that the middle of the red space, the whole of the orange, a great part of the green, a considerable part of the blue, a little of the indigo, and a very little of the violet, vanish, being absorbed by the blue glass; and that the yellow rays occupy a larger space, covering part of that formerly occupied by the orange on one side, and by the green on the other; so that the blue glass absorbs the red light, which, when mixed with the yellow, constitutes orange; and also absorbs the blue light, which when mixed with the yellow forms the part of the green space next to the yellow. Hence, by absorption, green light is decomposed into yellow and blue, and orange light into yellow and red. Consequently the orange and green rays, though incapable of decomposition by refraction, can be

resolved by absorption, and actually consist of two different colours possessing the same degree of refrangibility. Difference of colour, therefore, is not a test of difference of refrangibility, and the conclusion deduced by Newton is no longer admissible as a general truth. By this analysis of the spectrum, not only with blue glass but with a variety of coloured media, Sir David Brewster, so justly celebrated for his optical discoveries, has proved, that the solar spectrum consists of three primary colours, red, yellow, and blue, each of which exists throughout its whole extent, but with different degrees of intensity in different parts; and that the superposition of these three produces all the seven hues according as each primary colour is in excess or defect. Since a certain portion of red, yellow, and blue rays constitute white light, the colour of any point of the spectrum may be considered as consisting of the predominating colour at that point mixed with white light; consequently, by absorbing the excess of any colour at any point of the spectrum above what is necessary to form white light, such white light will appear at that point as never mortal eye looked upon before this experiment, since it possesses the remarkable property of remaining the same after any number of refractions, and of being capable of decomposition by absorption alone.

When the prism is very perfect and the sunbeam small, so that the spectrum may be received on a sheet of white paper in its utmost state of purity, it presents the appearance of a riband shaded with all the prismatic colours, having its breadth irregularly striped or subdivided by an indefinite number of dark and sometimes black lines. The greater number of these rayless lines are so extremely narrow that it is impossible to see them in ordinary circumstances. The best method is to receive the spectrum on the objectglass of a telescope, so as to magnify them sufficiently to render them visible. This experiment may also be made, but in an imperfect manner, by viewing a narrow slit between two nearly-closed window-shutters through a very excellent glass prism held close to the eye, with its refracting angle parallel to the line of light. When the spectrum is formed by the sun's rays, either direct or indirect, as from the sky, clouds, rainbow, moon, or planets, the black bands are always found to be in the same parts of the spectrum, and under all circumstances to maintain the same relative positions, breadths, and intensities. Similar dark lines are also seen in the light of the stars, in the electric light, and in the flame of combustible substances, though differently arranged, each star and each flame having a system

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of dark lines peculiar to itself, which remains the same under every circumstance. Dr. Wollaston and Fraunhofer of Munich discovered these lines deficient of rays independently of each other. Fraunhofer found that their number extends to nearly six hundred. From these he selected seven of the most remarkable, and determined their distances so accurately, that they now form standard and invariable points of reference for measuring the refractive powers of different media on the rays of light, which renders this department of optics as exact as any of the physical sciences. The rays that are wanting in the solar spectrum, which occasion the dark lines, are possibly absorbed by the atmosphere of the sun. If they were absorbed by the earth's atmosphere, the very same rays would be wanting in the spectra from the light of the fixed stars, which is not the case, for it has already been stated that the position of the dark lines is not the same in spectra from star-light and from the light of the sun. The solar rays reflected from the moon and planets would most likely be modified also by their atmospheres, but they are not,-for the dark lines have precisely the same positions in the spectra, from the direct and reflected light of the sun.

A perfectly homogeneous colour is very rarely to be found, but the tints of all substances are

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