466 



WRIGHT: PETROGRAPHIC MICROSCOPE 



centered, once for all, in a vertical direction by rotation in an 

 excentric steel supporting ring and in a horizontal direction by 

 one of the hardened steel screws A or B; against the flanges of 

 these screws the hardened steel face-plates of the carriage strike. 

 Mechanically the bearing surfaces are wide and the objectives 

 return to exact center and focus on changing. For most petro- 



graphic work 

 two objectives 

 only are re- 

 quired (high 

 power, E. F. = 

 4 mm., and low 

 power E. F. = 

 16 mm.) and 

 one carriage 

 suffices for the 

 purpose. But 

 occasionally ob- 

 jectives of other 

 focal length are 

 desired; these 

 are then mount- 

 ed on a second 

 slider which is 

 slipped into the 

 mount in place 

 of the first car- 

 riage. In this 

 operation the 

 semi-circular 

 flange of the 



stop screw B (fig. 1) is first turned through 90° to allow the 

 slider to pass. 



2. The removal of the astigmatism introduced by the analyzer. 

 It has long been known that the introduction of the analyzer 

 into the optical system seriously disturbs the optical quality of 

 the image by introducing into it astigmatism and other defects. 



Fig. 1, Petrographic microscope with new accessories: 

 A, sliding objective changer; B, lens system for removal 

 of astigmatism caused by analyzer; C, prism for observation 

 of interference figures; D, device for use in the accurate 

 measurement of extinction angles. 



