38 WRIGHT: A NEW CRYSTAL-GRINDING GONIOMETER 



intensity of illumination secured by this device is similar to that 

 of the Abbe refractometer slit, while the accuracy of settings is 

 considerably better than that of the angular readings, namely, I'. 

 The telescope is mounted on a vertical rotating axis and can be 

 raised by means of a pin and ratchet movement. {X, fig. 2, 

 M, fig. 3a.) Enlarged signals are obtained by means of objective 

 with eyepiece E; reduced signals, by objective P and objective 

 0; images of the crystal surface, by means of objective alone, 

 or by objective P + objective + eyepiece E. This telecentric 

 arrangement of the optical system has been found well suited to 

 the purpose. The mounted crystal is held by the clamp S (fig. 2), 

 is adjusted by the ball and socket device R, is centered by the 

 sliding carriages Q, and is moved up and down by the screw P 

 which operates the triangular steel stem indicated in the figure. 

 The theodolite position-angles can be read directly to |' on the 

 circles Ci and C2.* 



The grinding wheel (fig. 2) is driven slowly (200 r.p.m.) by a 

 small low speed | h.p. motor (400 r.p.m.), and is mounted on a 

 rotating axis W, supported between arms Y which allow the wheel 

 to be raised and lowered without tilting. The grinding wheel 

 itself is supported on an adjustable steel block. Experience has 

 shown that for grinding and polishing purposes block-tin wheels 

 are the most satisfactory. Three or more such wheels are in 

 constant use and are kept flat by grinding them, the one against 

 the other, in rotation. Pitch wheels have also been used for 

 polishing purposes but they are apparently not greatly superior 

 to the tin disks and are more troublesome to prepare. The 

 grinding disk is adjusted normal to its axis of rotation by means 

 of light rays reflected either from the upper surface of the wheel 

 itself or from a plane-parallel optical glass plate 3 inches in diame- 

 ter placed on top of the grinding disk. Only the finest abrasives 

 are used, crystolon or alundum 65 F of the Norton Company, 

 or washed emery M 303 of the American Optical Company. 

 Polishing powders are: washed rouge, putty powder, and chro- 

 mium oxide. These polishing powders are not equally good for 



* These circles were specially graduated by Bausch & Lomb Optical Company; 

 no error approaching j' in magnitude has been detected in the graduations. 



