ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



233 



of tbe pillar, and formed the rapid movement. The slow movement was 

 produced by a differential screw D passing through the first and in- 

 serted into the top of the rod E actuating the lever. The rod was 

 prevented from rotating by a slot and pin F. The differential screw 

 was rotated by the upper milled head A. When the upper head wa& 

 rotated and the lower untouched, the slow motion obtained was derived 

 from the action of the differential screw. If both were turned together, 

 the resulting movement would be derived from the lower head only. 

 This form of fine adjustment was designed for lever instruments in 

 which efficiency is the chief consideration. Its principal drawback is 

 that it involves the use of a differential screw, which would require to 

 be very accurately made. 



For many instruments with direct-acting fine adjustments, especially 



Fig. 41. 



instruments of the Continental type, a more simple plan is recommended. 

 In many Microscopes of this class the limb of the instrument is attached 

 to a sleeve, C in fig. 41, sliding up and down outside a prismatic bar 

 F, which is a fixed part of the stand. A micrometer screw D, secured 

 at its base to the top of the upright bar, is carried upwards through an 

 aperture in the cap, and the milled head A is really a loose nut work- 

 ing upon this screw, whilst the cap, limb, tube, &c. are forced up into 

 contact with the milled head by the action of a spring G. This very 

 simple adjustment may, by a single addition, be converted into one 

 having two rates of speed, it only being necessary to insert under the 

 ordinary head A, a second head B attached to the hollow screw E r 

 working into the cap. The latter screw is made hollow, so that it may 

 slide freely over the first screw D. The relative action of the two 

 milled heads in this case is as follows : — the upper head when turned to 



