438 Johannsen — A Petrographical Microscope. 



In the microscope as purchased, the nicol is held in place by 

 the pressure of a part of the tube between two saw cuts. 

 Before attaching the collar, this pressure arm is cut off just 

 above where the upper edge of the collar is to come, and* the 

 upper part is bent outward until only enough friction remains 

 to hold the nicol in any position to which it may be turned. 

 It is necessary to saw off the lower part of this section in order 

 that the upper part may later, if necessary, be bent more or 

 less to produce the proper amount of friction. On a new 

 instrument the cut would be made only above. The collar is 

 soldered on the tube so that the nicols are approximately 

 crossed when the lever is set at 0°. Accurate adjustment 

 may later be made by rotating the prism within the tube. 



In the Fuess microscope with rotating upper nicol, the 

 vibration plane is at right angles to the direction of the lever, 

 and the indicator points to 90° on the scale. Consequently, 

 the upper and lower scales have readings differing by 90° 

 when the nicols are crossed, and have the same readings when 

 they are parallel. 



In using certain accessories it is sometimes necessary to place 

 the ocular so that the cross hairs form angles of 45° with the 

 vibration directions of the nicols. With a microscope fitted 

 as above, both nicols may be rotated to 45° and the ocular left 

 in its normal position. 



1±. Additions to the Hirschwald stage. — Figure 4 shows two 

 scales engraved upon the two parts of the Hirschwald stage. 

 A mark on the sliding portion indicates the distance through 

 which the plate has been moved. Horizontal movement is 



registered by a small scratch made 

 with a diamond point on the lower 

 margin of the thin section. Any 

 mineral, whose position is once reg- 

 istered, may again be located by 

 resetting the stage to the former read- 

 ing. By reading first the vertical and 

 then the horizontal scale, the stage 

 setting shown in the diagram is in- 

 dicated by the figures "1-5-2-0" (if 

 the scratch on the slide is at the 

 letter a). 



The stage, as made at present, does 

 not permit a high power objective to be brought close to a 

 slide of normal thickness when the mineral is near either of 

 the clamping bars (b, V). The bars should be beveled slightly 

 at a and a'. In a corresponding position on the under side, 

 enough of the sliding stage should be cut away to permit the 

 condensing lens to be carried against the slide at its edges as 

 well as at the center. 



The University of Chicago, February, 1910. 



Fig. 4. 



b 



jOl 



J© 



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a* 

 a 





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b 







