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Spectrum Analysis
IN the chapter upon Optics, we have already seen that, if a prism be placed against a hole in the shutters of a darkened room through which a ray of sunlight is projected, the white light is broken up or decomposed into its constituent colours. This experiment was first made by the great Sir Isaac Newton, and made, too, in this identical manner. Writing to the Royal Society, the great philosopher says :--"I procured me a triangular glass prisme to try therewith the celebrated phenomena of colours. And, in order thereto having darkened my chamber and made a small hole in my window-shuts to let in a convenient quantity of the sun's light, I placed my prisme at his entrance, that it might thereby be refracted to the opposite wall." The effect upon the opposite wall seems to have greatly surprised Sir Isaac, for instead of one round spot of light he saw seven discs of colour, one above the other, but each one overlapping and mixing with its neighbour. The lowest disc was red, then orange, yellow, green, blue, indigo, and lastly came the uppermost disc which was violet. In this way Newton discovered that the white light of the sun was really compounded of rays of all colours; and we have already seen that a round card painted with these colours in certain proportions will, when rapidly turned, appear to be white. The next important step in the history of the spectroscope was the discovery by Wollaston at the beginning of this century. He found that if, instead of a round hole as used by Newton, a slit were substituted, the colours no longer appeared as discs, but as a beautiful ribbon. At first he imagined that the ribbon was continuous, but on closer examination he found that it was interrupted by Solar Spectrum, and Spectra of the Alkalies and Alkaline Earths. Copied from the Original Drawings of G. KIRCHHOFF and R. BUNSEN. innumerable dark spaces--fine lines, like black hairs, which cut the ribbon right across. Some years later a German optician named Fraunhofer also observed these lines, and noticed that they always occupied certain positions in the different colours of the spectrum. He made a map of them, calling those which were the more prominent by the letters of the alphabet--A, B, C, D, E, F, G, and H. In compliment to him these lines are still known as Fraunhofer's lines, and the letters which he attached to them for comparison and reference are still retained. The lines A, B, and C are in the red part of the spectrum, D is in the yellow, E in the green, between this and the blue is F, then comes G in the violet-blue, and lastly H in the violet. In addition to these principal lines Fraunhofer's map showed nearly 600. By more perfect apparatus these have been increased, until the number of lines in the solar spectrum is now upwards of 2,000.
Neither Fraunhofer nor Wollaston succeeded in divining the meaning of these strange marks of darkness. But the former made a curious observation, which helped experimenters later on to solve the problem. He found that, when the prism was subjected to the light from burning sodium, an exceedingly bright yellow line appeared in place of the dark line D. Any one who has a spectroscope can easily repeat this experiment by placing a few grains of common salt (chloride of sodium) upon the wick of a spirit lamp. When the instrument is first turned towards the sky, the position of the dark line D is plainly seen. Upon turning it towards the spirit flame, a brilliant yellow line immediately takes its place. In the same way it has been found that the bright lines given by different chemical elements found their counterpart in dark lines on the solar spectrum. The reason why these lines are dark in the latter case is expressed in the following law :--"Every gas and every vapour absorbs exactly those kinds of rays which it emits when in a glowing condition, whilst it permits all other kinds of rays to traverse it with undiminished intensity." So that we may imagine that the cool vapour of sodium in the atmosphere surrounding the sun absorbs the rays from the incandescent sodium burning in the body of the sun; therefore we get a dark line instead of a bright one.