about two inches. At its focus, in H, and in nearly the same plane as the
face of the slit, is a single vertical silk fiber. The apparatus is
furnished with a standard with rack and pinion, and the base furnished
with leveling screws.



Manner of Using the Micrometer.


In measuring the deflection, the eye-piece is moved till the cross-hair
bisects the slit, and the reading of the scale and divided head gives the
position. This measurement need not be repeated unless the position or
width of the slit is changed. Then the eye-piece is moved till the
cross-hair bisects the deflected image of the slit; the reading of scale
and head are again taken, and the difference in readings gives the
deflection. The screw was found to have no lost motion, so that readings
could be taken with the screw turned in either direction.



Measurement of Speed of Rotation.


To measure the speed of rotation, a tuning-fork, bearing on one prong a
steel mirror, was used. This was kept in vibration by a current of
electricity from five "gravity" cells. The fork was so placed that the
light from the revolving mirror was reflected to a piece of plane glass,
in front of the lens of the eye-piece of the micrometer, inclined at an
angle of 45°, and thence to the eye. When fork and revolving mirror are