The cone bearings are of selected materials and the upper and lower parts of the arms are turned and ground in one operation giving the most accurate alignment. Clamps for telescopes and prism table are designed so that no strain comes on the cone-bearings from clamping, thus avoiding any disturbance of the axial alignment. The cone has a carefully determined angle which gives precision centering and at the same time does not give excessive lateral pressure. Scale is 10" from lifetime stainless, divided on the latest dividing machines. Accuracy of calibration within + 0.01%. Adjustment is provided to verniers for very fine opposition to the scale. The slit jaws are from stainless steel, finely ground and the jaw movement is free of any spring action ensuring perfect parallelism at all openings. Vernier Reading 10 sec