Petrogva/phical Microscope. 



437 



reason the arrangement described below has been found very 

 effective. 



A brass collar was turned to fit the lower part of the nicol tube 

 and was soldered on as shown in figure 3. A section through 

 the collar is shown at A, figure 3 ; the nicol tube being here 



m 



* m u 



c3 



Fig. 3. 















i 



b 



.. 



a 







a| 



*| 



; 













90' 



1 , , 



45* 



, ,1, , 







; 





r^\- 





i\ 



Jc= 





' 







D 



indicated by dotted lines. B is a view from below ; C, a view of 

 the inside of the collar as it would appear if straightened out ; 

 and D, the outside of the collar similarly unrolled. A groove 

 is cut on the inside to receive the head of a screw projecting 

 from the side of the nicol tube (b b', fig. 3, B and C). On the 

 opposite side of the tube, a lever (c) moves in the slot a a" 

 (fig. 3, B), which is of such length that the distance between 

 centers of the lever, in the positions a' a", is just 90°. The 

 screw head and the lever bar thus form the bearings to carry 

 the nicol tube. As the lever is moved from a' to a", 

 the screw head b slides in the groove from b to V . (As a 

 matter of fact this groove was turned the entire circum- 

 ference of the collar, for convenience of construction, though 

 it is not so shown in the diagrams.) The nicol tube may be 

 removed or inserted easily by slightly raising and rotating the 

 tube until the lever bar passes over the projection at a 

 (fig.3,D). 



