4:10 F. F. Wright — New Petrographic Microscope. 



independently of the upper nicol. The total angle through 

 which both nicols can be revolved by this device is 190°.* 



(2) A mechanical stage of new designf (fig. 3). This 

 stage is practically dust-proof and mechanically simple in con- 

 struction. In fig. 3. the vertical edge or rim of the cap, O, 

 of the stage plate is indicated by the shaded broken circle, the 

 upper surface of this plate being considered removed and the 

 working parts as seen from above thus exposed to view. The 

 small plates I x and T 2 are attached to the lower stage and 

 are so constructed that wedge-shaped edges allow the 

 rectangular plate K to move only in an east-west direction. 

 This movement is effected by means of the screw H 2 . The 

 plates J l5 J 2 and J 3 of fig. 3, on the other hand, are attached 

 to the upper movable plate O, and their wedge-shaped edges 

 are so adjusted that they allow the upper plate to move only 

 in a north-south direction with reference to the rectangular 

 piece K. The screw H, which terminates in a block attached 

 to the upper plate and running in a sliding pin D, accomplishes 

 these north-south movements. The heads of both screws H, 

 and H 2 have divisions reading to *01 mm movement. Springs 

 not indicated in the figure have been introduced and oppose 

 the forward motion of the screws H a and H 2 and thus obviate 

 errors due to lost motion in the screws. The total movement 

 of the stage plate in any direction is 2i mm . Mechanically, it is 

 of simple construction and consists of few parts. 



(3) The metal part containing iris diaphragm and polarizer 

 can be withdrawn from the optic axis of the microscope by 

 means of a release spring not shown in fig. 1. This part is 

 also revolvable by itself about the axis. This arrangement 

 was adopted in preference to the usual method of inserting 

 and withdrawing the upper nicol because of the disturbing 

 effect which the introduction of the upper nicol causes, both on 

 the focus and position of the field. With the present disposi- 

 tion, the upper nicol \ remains permanently in the upper tube 

 and the optical system, objective, nicol, and ocular, is not 

 disturbed in passing from ordinary to polarized light. In 

 certain microscopes the effect of the upper nicol on change of 

 focus is compensated by means of a small lens of weak magni- 

 fying power, but even after the introduction of this device 

 some shifting of the field may still be experienced on inserting 

 the upper nicol. 



*Fuess & Company have recently constructed, at the writer's suggestion, 

 a simplified model of microscope on this principle of a rigid bar connection 

 between the two nicols, so that the two can be revolved simultaneously. 

 (Price 300 mks.) 



f This stage was designed by Mr. Chamberlin of this laboratory, and con- 

 structed by Mr. Semple. 



% For certain positions of the reflector and on certain days the light from 

 the reflector is polarized to such an extent that faint polarization colors are 

 observed on minerals in the thin section even after the polarizer has been 

 withdrawn. For this reason it would be an improvement if the upper nicol 

 also could be withdrawn readily, whenever desirable. This is the case on 

 the model constructed by Fuess & Co., noted above. 



