F1.NE ADJUSTMENTS 33 



in this case is conveyed from the screw to the slide 

 through an intermediate lever or other device which 

 makes the movement slower. Such speeds as ^inr or -svn 

 of an inch for each revolution of the milled head . are 

 usual. A lever is the simplest and one of the most perfect 

 devices, and the ratio of the lengths of its arms govern^ 

 the rate of motion. One rotation of a screw of -gV' pitch 

 bearing on the longer arm of a lever whose ratio is 5 : 1 

 raises the other end ^j-^ of an inch, and so moves the tube 

 through that distance. A lever of this description is 

 placed in the arm of the Van Heurck and Edinburgh 

 microscopes (Figs. 13 and 18). Other makers adopt other 

 devices for reducing the motion imparted by the micro- 

 meter screw. Zeiss, for instance, employs Berger's 

 micrometer screw, and Leitz a cam whose rotation raises 

 the tube or allows it to fall. Figures and descriptions of 

 these are given in the respective catalogues. 



A fine movement of this description is essential for 

 the highest power work, but for low and medium powers 

 a quicker movement is easier to work with, and -V' to 

 TI^O" is quite adequate. If the milled head is graduated, 

 its rotation may be used for measurements in a vertical 

 direction, provided the pitch of the screw is known, or 

 the movement caused by one complete rotation is given 

 by the maker. 



The position of the actuating milled heads is not 

 of very great importance, provided that they can be 

 easily manipulated while the hand or arm rests on the 

 table. The lateral position is ver}^ popular now, and when 

 using it the hand may be rested on the tripod foot, but 

 it is somewhat easier to connect a milled head situated 

 on top of the limb to a focussing red. 



Testing the Fine Adjustment. Any of the forms or 

 modifications of fine adjustment adopted by makers of 

 repute can be relied on to give satisfaction, but in testing 

 a microscope it is well to pay particular attention to 



