ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



503 



shown in fig. 53, fig. 54 being a cross section. The chief changes which 

 have been introduced are as follows. 



1. The nicols are revolvable simultaneously about the optic axis of the 

 Microscope. They are connected rigidly by the bar T, and their angle of 

 revolution can be read off directly by the stage-vernier. This method 

 obviates the errors introduced by the usual system of gear-wheels, with 

 accompanying lost motion in the moving parts. The details of construc- 



Fig. 54. 



Fig. 55. 



tion are shown in fig. 54. By means of the screw and cross bar at B 

 the connecting rod T can be instantly released, and the lower nicol 

 withdrawn or revolved by itself independently of the upper nicol. The 

 total angle through which both nicols can be revolved by this device is 

 190°. [The author afterwards learned that the device of a connecting 

 rod was not altogether new, having been used by Dick over thirty years 

 ago, but was applied only to the revolution of a cap nicol above the 

 ocular in conjunction with the polariser.] 



2. A mechanical stage of new design (fig. 55), practically dust-proof 



