Page:Elementary Text-book of Physics (Anthony, 1897).djvu/417

§ 332] for it to pass over a small distance on the earth's surface. In the form of experiment devised by Fizeau, a beam of light is allowed to pass out through a small hole in the shutter of a darkened room to a distant station, where it is reflected back on itself. It returns through the opening and produces an image of the source. A toothed wheel is placed in front of the opening in such a position that, to pass out or back, the light must pass through the spaces between the teeth. If the wheel revolve slowly, as each space passes the opening in the shutter light will pass out, and returning from the distant station will enter through the space by which it made its exit. An image of the source will therefore be visible whenever a space passes the opening, and in consequence of the persistence of vision this image will appear continuous. Since it takes time for the light to go to the distant station and back, it is possible to give to the wheel such a velocity that when the light which passed out through a given space returns, it will find the adjacent tooth covering the opening, so that no image of the source can be seen. If the velocity of rotation be sufficiently increased, the image again comes into view when the light can enter through the space following that by which it emerged. A still further increase of velocity may cause a second extinction of the image. The experiment consists in determining accurately the velocities for which the several extinctions and reappearances of the image occur. A high degree of accuracy cannot be attained because the extinction of the image is not sudden. It disappears by a gradual fading away, and reappears by a gradual brightening. For quite a range of velocity the image cannot be seen at all.

332. Foucault's Method.—Foucault's method depends upon the use of the revolving mirror as a means of measuring a very small interval of time. Foucault's experiments were repeated with some modification by Michelson in 1879 and again in 1882. The general theory of the experiment may be understood from the following brief description: Let $$S$$ (Fig. 97) be a narrow slit, $$m$$ a mirror which may revolve about an axis in its own plane, $$L$$ a lens, and $$m'$$ a second mirror. Light from a source behind $$S$$ passes through the slit,