Page:Light waves and their uses.djvu/78

 LECTURE IV

THE APPLICATION OF INTERFERENCE METHODS TO SPECTROSCOPY

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, yellow, 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 sunlight 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 discoverer Fraunhofer's lines, have this very important characteristic, 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