ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
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beneath the Bertrand lens. The rotation of the nieol is effected by 
the head d (fig. 28), and the angle is read off on the divided quadrant T. 
The two principal positions (0° and 90°) are marked by stops. 
Accessory Apparatus . — Of especial importance in mineralogical in- 
vestigations for the determination of differences of refraction in rock 
constituents is the iris-diaphragm. Fig. 29 shows how the iris- 
diaphragm beneath the stage is connected with the holder of the polariser. 
