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-332] Lighthousc Lenses.
329 Paris Universal Exhibition in 1855. The system of mirrors and lenses alone is io feet high.
A lighthouse lens of this kind sends a powerful beam of light towards eight points of the horizon, but all other points are desti
tute of light, so that vessels sailing in these dark parts would have no help from the lighthouse. This difficulty was removed by Fresnel by means of a very simple mechanism, represented at the lower part of fig. 258. A clockwork motion, M, moved by a weight, P, imparts to the whole system of lenses, AB, a slow rotating motion on six rollers. During a complete revolution of the apparatus, the
whole horizon is successively illuminated, and the mariner lost in the night sees the light alternately appear and disappear after equal intervals of time. These alternations serve to distinguish lighthouses from an accidental fire or a star. By means too of the number of times the light disappears in a given time, and by the
colour of the light, sailors are enabled to distinguish the lighthouses from one another, and hence to know their position.
Of late years the use of the electric light has been substituted for that of oil lamps; a description of the apparatus will be given in a subsequent chapter.
CHAPTER V. DECOMPOSITION OF LIGHT BY PRISMS. 333. Solar spectrum.-In speaking of prisms and lenses, we have only considered the change in direction which these transparent media impart to luminous rays, and the images which result 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 lights, a phenomenon to which the name dispersion is given.
In order to show that white light is decomposed by refraction, a pencil of solar light, SA (fig. 260), 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 at K; but if a fint 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, 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 adjoined 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, is not 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.
334. 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 a 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. Fresnel has calculated that 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 millimeter, and for violet 425 millionths.
335. Luminous, calorific, 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 calorific action of the spectrum is demonstrated by successively placing a very delicate thermometer in the various parts of the spectrum. It is observed that in the red, or even a little beyond it, the heat attains its greatest intensity. This proves the existence of invisible heat rays, which are less refrangible than all other spectral rays.
Passing from the calorific action of light to its chemical action, we may first observe 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 : there are gaseous mixtures, also, which suddenly explode when exposed to the sun's rays. These chemical effects are not produced equally in all parts of the spectrum; the greatest chemical action is met with in the violet, and even a little beyond.
We may thus say that the heating effect is met with in the extreme red, the luminous in the yellow, and the chemical action in the extreme violet.
336. Dark lines of the spectrum.—The colours of the solar spectrum are not perfectly continuous : throughout the whole extent of the spectrum are a great number of very narrow dark lines. They are best observed by admitting a pencil of solar rays into a darkened room through a narrow slit. If at a distance of three or four yards we look at this slit through a flint glass prism, with its edge held parallel to the edge of the slit, we observe a number of very delicate dark lines parallel to the edge of the prism, and at very unequal intervals.