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CHAPTER V.

DECOMPOSITION OF LIGHT BY PRISMS.

356. Solar spectrum.-In speaking of prisms and lenses, we have only considered the change in direction, which these transparent media produce in luminous rays, and the images which result

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therefrom; but the phenomenon of refraction is by no means so simple as we have hitherto assumed: when white light, or that which reaches us from the sun, passes from one medium into another, it is decomposed into several kinds of light, a phenomenon to which the name dispersion is given.

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The Colours of the Spectrum.

⚫377 In order to show that white light is decomposed by refraction, a pencil of solar light (fig. 313) is allowed to pass through a small aperture in the window shutter of a dark chamber. This pencil tends to form a round and colourless image of the sun on a screen; but if a flint-glass prism arranged horizontally be interposed in its passage, the beam, on emerging from the prism, becomes refracted towards its base, and produces on a distant screen a vertical band, rounded at the ends, coloured in all the tints of the rainbow, which is called the solar spectrum. In this spectrum, the production of which forms one of the most brilliant optical experiments, there is in reality an infinity of different tints, which imperceptibly merge into each other; but, with Newton, it is customary to distinguish seven principal colours, as seen in the coloured plate. These are violet, indigo, blue, green, yellow, orange, red: they are arranged in this order in the spectrum, the violet being the most refrangible, and the red the least so. They do not all occupy an equal extent in the spectrum, violet having the greatest extent, and orange the least.

From the experiment of the solar spectrum Newton concluded that white light—that is, light coming from the sun-is not homogeneous (that is, simple), but consists of seven different lights, which, united, give the impression of white, while, when separated, each produces its own colour. He ascribed the separation of these seven lights on their passage through the prism to their different degrees of refrangibility. For if they were all equally refrangible, as they would be equally bent on entering and emerging from the prism, they would traverse it without being separated, and the light would be white on emerging as well as on incidence.

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357. The colours of the spectrum are simple. If one of the colours of the spectrum (the yellow, for instance) be isolated by intercepting the others by means of an opaque screen, and if the light thus intercepted be allowed to pass through a second prism, it is deflected, but without decomposition; that is, it only gives rise to a single emergent pencil. As the same phenomenon is observed with the other colours of the spectrum, it is concluded that they are indecomposable by the prism, which is expressed by saying; that the seven colours of the spectrum are simple or primitive colours.

As regards the cause in virtue of which one part of the spectrum produces on us the sensation of red, another of yellow, another of orange, and so forth, the undulatory theory teaches us that it depends

upon the number of vibrations performed by the molecules of ether. This number, which is very great, differs with each colour, and increases from red to violet; for the extreme red it is 458 millions of millions in a second, and for violet 727 millions of millions. As the velocity of propagation is the same for all the colours of the spectrum, but each corresponds to an unequal number of vibrations, it follows that the length of these vibrations must vary with different colours. It has been calculated that, in the case of red, the length of the vibration is 620 millionths of a millimetre, and for violet 425 millionths.

358. Luminous, heating, and chemical effects of the spectrum.—The various spectral rays differ not only in their colour but also in their luminous power, in the heat by which they are accompanied, and by the chemical effects to which they give rise. It is found that the middle pencils, the yellow and the green, illuminate the most powerfully. Thus the print of a book placed in the yellow pencil is seen more distinctly than in the red or violet.

The heating action of the spectrum is demonstrated by successively placing a very delicate thermometer in the various parts of the spectrum. It is observed that the heat attains its greatest intensity in the red, or rather a little beyond it. The existence of these invisible heat rays, which are less refrangible than all other spectral rays (221), was discovered by Sir J. Herschel, from which fact they are called Herschellian rays.

Passing from the heating action of light to its chemical action, we find that it tends to destroy most vegetable colours, such as wall papers and dyed stuffs, which rapidly fade if exposed to bright light. Some chemical substances are known which are naturally white, and are blackened by the luminous rays, on which property depends the art of photography: there are gaseous mixtures, also, such as that of hydrogen and chlorine, which suddenly explode when exposed to the sun's rays. These chemical effects are not produced equally in all the parts of the spectrum; the greater chemical action is met with in the violet, and even a little beyond.

Figure 314 represents the distribution of the heating, the uminous, and the chemical action of the spectrum; the shaded lines representing the parts of the spectrum which are not visible to the eye, and which, it will be seen, are about equal in length to the luminous parts. The curve I represents the heating effect of the spectrum, from which it will be seen that it is greatest at a little distance outside the visible red; the curve II represents the

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