Fig. 361.—End of rod mounted with microscopes, trestles and ground plate.
Larger image
The bars are placed in strong wooden boxes, to the bottoms of which are fixed the plates that hold the brass rollers upon which the bars are supported, Fig. 362 F, and the central stay E mentioned before prevents any displacement of the bars when the rods are held by the rollers RR′. To protect the tongue A, which projects beyond the boxes, there is a special covering nozzle having a hole and cover over the dot. A level is placed on one of the bars, which is seen through a window in the lid of the box. At the ends of the box plates are fixed for supporting the tripod of the double compensated microscope, Fig. 361 D, employed to observe the standard points of one pair of rods brought by adjustment to true position MM′. A pair of sight vanes which shut down are placed on the ends of the box for setting the rods approximately in line.
Colby Compensated Measuring Rods.
Fig. 362.—Side elevation of point of support of rod.
Fig. 363.—Side elevation of centre, with section of box bx.
Fig. 364.—Plan of rods and compensating arm.
Fig. 365.—Perspective view of the same.
Larger image