Fig. 1.—Ground plan, showing arrangement of mirrors
50 horse-power motor, reduced the pressure in the pipe to about a half-inch in three hours. Most of the measurements were made when the residual air in the pipes had been reduced to a pressure of about half an inch to one inch of mercury. At these pressures the fringes were perfectly steady, and as sharply defined as could be desired.
Fig. 2.—Details of comer box and mirror mounting
The fringes to be measured were those formed by the beams going in opposite directions about the circuit ADEF. As a fiducial mark from which to measure the displacement, a second set of fringes was formed by the mirror system ABCD. The area inclosed by this circuit was much too small to give a measurable displacement of the fringes, and the shifts actually recorded were those between the central fringes of the two sets.