228 



Transactions of the Society. 



have therefore adopted the system of spring pincers of Professor 

 Thury — the adjusting ring allows an exact centering and the 

 application of the objective is effected instantaneously. 



Fig. 15. 



The stage of the instrument has a circular division with 

 a vernier for the exact determination of the point of extinction in 

 the plane of polarization. It also has two transverse divisions 

 serving as a finder as well as a second rotating plate, the utility of 

 which is not absolute and is only suitable for special mineralogical 

 observations. 



