Page:Light waves and their uses.djvu/78

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LECTURE IV

THE APPLICATION OP INTERFERENCE METHODS TO

SPECTROSCOPY

Doubtless most of us, at some time or other, have looked through an old-fashioned prismatic chandelier pendant and observed that when held horizontally it produces the very curious effect of making objects appear to slope downward as though going down hill; and certainly you have all noticed the colored border which such a pendant produces at the edge of luminous objects. This experiment was made first under proper conditions by Newton, who allowed a small beam of sunlight to pass through a narrow aperture into a dark room and then through a glass prism. He observed that the sun's image was drawn out into what we call a spectrum, i. e., into a band of colors which succeed one another in the well-known sequence — ■ red, orange, yel- low, green, blue, violet; the red being least refracted and the violet most.

If Newton had made his aperture sufficiently narrow and, in addition, had introduced a lens in such a way that a distinct image of the slit through which the sun- light passed was formed on the opposite wall, he would have found that the spectrum of the sun was crossed by a number of very fine lines at right angles to the direction in which the colors extended. These lines, called after the dis- coverer Fraunhofer's lines, have this very important char- acteristic, that they always appear at certain definite positions in the spectrum ; and hence they were used for a considerable time for describing the location of the different colors of the spectrum. We shall endeavor roughly to present this

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