8 EXPERIMENTS WITH THE DISPLACEMENT INTERFEROMETER. 



FI, A'J, C'K provide for firmness in the horizontal direction. The feet 

 ABC and A'B'C' were additionally cross-braced (not shown). This truss is 

 not ideal ; but as it had to pass through a square hole in the floor of the room, 



FIG. 6. 



50 cm. above the cement floor, conditions had to be compromised. It seemed, 

 however, to meet the initial requirements of the experiment adequately. All 

 rods are of X-i ncn gas-pipe, usually screwed fast at one end and clamped at 



FIG. 7. 



the other. The position of the horizontal pendulum on a separate mounting 



is sketched in at hPh'P', the glass plate being at h and h', the pivots at p and p'. 



The two adjustable opaque mirrors are shown at M and N, the former being 



about 20 cm. to the rear, and held by clamps. 5 is the slit and L the lens of 



