38 Light Waves and Theib Uses
refraction we may obtain a very considerable number of variations of form, as illustrated in Figs. 34, 35.
One of these types, enlarged in Fig. 36, has been arranged
Microscope, Telescope, Interferometer 39
in such a way as to show the extreme delicacy of the interferometer in measuring exceedingly small angles. For this purpose two of the
mirrors, C and D, have been mounted on a piece of steel shafting P two inches in diameter and six inches long. When the length of the paths of the two pencils is the same to within a few hundred thou- g sandths of an inch, the interference fringes in white light are readily observed, or may be projected on the screen. If, now, the steel shafting be twisted, one of the paths is lengthened and the other diminished, and for every
movement of one two-hundred-thousandth of an inch there would be a motion of the fringes equal to the width of a
FIG. 35
fringe.
Nc
tak
ing the end of the shafting be-
FlG. U7
steel
tween thumb and forefinger, the exceedingly small force which may thus be applied in this way is sufficient to twist