Application of Interference Methods 59
thousandth of a millimeter has been introduced, and projecting the interference fringes upon the screen, it will be noted that while ten or twelve of these fringes move past the fiducial line the spot of light will move over a corresponding distance on the scale. In moving through ten fringes the spot of light moves through six of the divisions, and therefore the length of one wave would bo six-tenths of a micron, which is very nearly the wave length of yellow light. If the light passes through a piece of red glass, and the ex{>eriment is repeated, the wave length will be greater; it is nearly sixty-seven hundredths. It is easy to see how the process may be extended so as to obtain very accurate measurements of the length of the light wave.
SUMMARY
1. A comparison between the corpuscular and the undulatory theories of light shows that the speed of light in a medium like water must be greater than in air according to the former, and less according to the latter. In spite of the inconceivable swiftness with which light is propagated, it has been possible to prove experimentally that the speed is less in water than in air, and thus the corpuscular theory is proved erroneous.
2. A number of applications of the interferometer are considered, namely, (a) the measurement of the index of refraction; (6) the coefficient of expansion; (c) the coefficient of elasticity; (cZ) the thickness of the ublack spot;'’ (e) the application to the balance; (/) the testing of precision screws; (g) the measurement of the length of light waves.
LECTURE IV
THE APPLICATION OF 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, /. 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 tl^e 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
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