5S Light Waves and Their Uses ferometer, with a corresponding increase in the delicacy of the test. I will conclude by showing how to measure the length of light waves by means of the interferometer. By turning m FIG. 53 the head attached to the screw, one of the interferometer mirrors (namely C, Fig. 39) can be moved very slowly. This motion will produce a corresponding displacement of the interference fringes. Count the number of interference fringes which pass a fixed point while the mirror moves a given distance. Then divide double the distance by the number of fringes which have passed, and we have the length of the wave. Using a scale marked from 0 to 10. made of such a size and placed at such a distance that, when a beam of light reflected from a mirror attached to tin* screw moves over one division, a difference in path of one- | 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 |