in such a way as to show the extreme delicacy of the interferometer in measuring exceedingly small angles. For this purpose two of theFIG. 36 mirrors, C and D, have been mounted on a piece of steel shafting P two inches in diameter and six inches in long. When the length of the paths of the two pencils is the same to within a few hundred thousandths 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 everyFIG. 37 movement of one two-hundred-thousandth of an inch there would be a motion of the fringes equal to the width of a fringe. Now, taking the end of the steel shafting between thumb and forefinger, the exceedingly small force which may thus be applied in this way is sufficient to twist