ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
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guiding screw, and is provided with a toothed wheel which engages in 
another toothed wheel attached to a so-called “ Spitzenfutter ” screwed 
on the spindle of the lathe. The screw is then cut slowly and carefully 
by the tool C (fig. 21), consisting of a three-sided prism, which can be 
easily removed for sharpening by loosening the steel wedge B, and can 
then be replaced in the holder A with the cutting plane in exactly the 
same position as before. 
The author also describes the so-called “ Spiegelfiihlhebel,” an 
instrument which may be used for all purposes of very fine adjustment 
and measurement, as e. g. in the fine-adjustment of large Microscopes. 
Fig. 19. 
Fig. 20. Fig. 21. 
The micrometer screw moves in a nut which is regulated by screws, 
and is terminated by a small, hard, highly polished ball. This screw 
carries a nut, on the edge of which is cut a screw-thread (usually 100 
teeth) in which an endless screw engages. This endless screw is fixed 
m a metal piece, and carries on its head a division ; so that with this 
arrangement 1/1000 of a screw rotation can be measured, i. e. in this 
case 1/20,000 mm. The screw is not in direct connection with the 
