404 T. A. Jaggar, Jr. — Microsclerometer, for 



a worm at the side. The foot-block F fits the left prong of 

 the U-shaped microscope foot. The distance to which it may 

 be slid on this prong is limited by a set-screw n, and a second 

 thumb-screw t binds it in place. The outer tube T, with all the 

 apparatus which it supports, rotates on the surface of the foot- 

 block F, and this rotation may be given fine adjustment by the 

 graduated barrel and screw R which presses against a projec- 

 tion o from the foot plate f of the outer tube. The vertical 

 rotation-axle passing through F may be made rigid in any 

 position by the thumbscrew t x . A Y-shaped extension of the 

 foot-plate f holds the sleeve in which rotates the main pulley- 

 bearing shaft q. The small pulley at the lower end of this 

 shaft is connected by a belt with the power, represented in 

 Plate XII as a disc and crank for rotation by hand. In the 

 author's latest experiments a clockwork motor has been used. 



The Balance Beam. — The beam, h, pivoted at a, is counter- 

 poised by a weight adjustable by the screw, c. The beam con- 

 sists of an upper and lower lath of brass, the space between 

 being destined to receive the pulley system for rotating the 

 diamond. The indicator arm bears an adjustable pendular 

 counterpoise 7 ounteract the effect of the 



arm passes through a slot 

 »wer plate • , y. 



land. — The diamond used is a cleavage tetrahe- 

 dron with a perfect point. The cleavage tetrahedron may be 

 obtained in duplicate in great perfection among the " cleav- 

 ings " used for making diamond cutters aud pencils. The 

 tetrahedra are especially valued for their pure points. As each 

 tetrahedron has four points, it is not difficult to find a point of 

 perfect form among relatively few specimens. The points are 

 turned upward in the field of the microscope successively, 

 under a high power (No. 7) until one is found which shows 

 the three edges to the uppermost focus of the instrument, 

 converging to a perfect point ; the diamond selected is centered 

 in its brass mount by soldering it in first roughly, and then 

 turning down the brass in a jeweler's lathe about the diamond 

 point as a center. The diamond D thus mounted is held in a 

 chuck by three radial centering screws. This chuck is attached 

 to a vertical steel spindle which passes through the pulley, p„ 

 and its pinion, and terminates in a jewel bearing against a 

 smooth diamond plate in the upper lath of the beam, just 

 under the point midway between the two weight pans, w. 

 The pans are separated in this fashion and two weights used, 

 in order to leave space for the microscope objective, which, as 

 will be seen, is focussed down close to the micrometer scale, m. 

 The superficial angle made by the two edges of each face of 

 the diamond at its point was measured in the microscope by 



