A. ASHE ON TWO-SPEED FINE ADJUSTMENTS. 133 



that it consisted of a hollow screw of comparatively coarse 

 pitch (g), to which the lower milled head (b) was attached. 

 This worked in the cap (c) on the top of the pillar, and formed 

 the rapid movement. The slow movement was produced by a 

 differential screw (d) passing through the first and inserted 

 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 was 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 action can easilv be seen from an 

 inspection of the figure. Its principal drawback is that it 

 involves the use of a differential screw, and would require to be 

 very accurately made. 



It is, however, apparent that the same principle can be carried 

 out in different ways, and that if a more simple form, without 

 a differential screw, can be made which will suit instruments with 

 direct-acting fine adjustments, especially those of the Continental 

 type, the value of the adjustment will be greatly increased, and 

 its field of usefulness proportionately extended. In many of these 

 instruments the limb of the microscope is attached to a sleeve 

 (c in Fig. 2), 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 working upon this screw, whilst the cap, 

 limb, tube, etc., 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, Fig. 2), a second 

 head (b) attached to the hollow screw (e), 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 the right operates 

 downwards, lowering the focus by depressing the sleeve (c) 



