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 be 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 experiment 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.
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; (b) the coefficient of expansion; (c) the coefficient of elasticity; (d) the thickness of the "black spot;" (e) the application to the balance; (f) the testing of precision screws; (g) the measurement of the length of light waves.