138 Transactions of the Society. 



Stops would be inserted on the summit of a third tube, sliding 

 easily within the iris tube, which should be lined with black cloth. 

 This would permit of the stop being adjusted in a vertical direction. 



In addition to these advantages, such a condenser mount 

 would have no projecting levers to foul the substage slides when 

 used in a rotating substage. 



The coarse adjustment to the body has been actuated from the 

 earliest days by rack and pinion, which has undergone various 

 improvements, such as Nelson's " stepped " rack, and finally the 

 oblique spiral form. 



This has several disadvantages. First, in laboratories the 

 exposed rack frequently gets dirty and damaged. Secondly, the 

 motion is rather more rapid than is desirable, more especially 

 when most of the work is done with high powers. This in turn 

 leads to the abuse of the fine adjustment commented upon by 

 every skilled microscopist when watching the average laboratory 

 worker. 



I propose that the rack and pinion be replaced by a multi- 

 thread large diameter screw, such as is used in the substage of 

 many student's microscopes, the body to travel on the usual 

 bearings. 



The milled head controlling the motion might be mounted on 

 the top of the limb or at the bottom, in the position adopted for 

 the fine adjustment in Baker's Nelson model. If lateral milled 

 heads were considered essential they could be introduced by 

 employing a similar system to that used by Zeiss in the Berger 

 pattern fine adjustment, substituting bevel for worm gear perhaps. 



The multi-thread screw would rotate in the microscope limb, 

 no vertical movement taking place, and movement would be 

 translated to the body tube by means of a split nut attached to 

 the latter and travelling upon the screw. 



The rapidity of the motion could be made as desired, but half 

 the average present rate is suggested as suitable. 



The advantages claimed for this system are : — 



1. Slowing down of the movement. 



2. No tendency to " run down " from the weight of the 



body or pressure on the draw tube. Hence an easier 

 working adjustment can be made than where the 

 pinion has to act the part of a brake. 



3. Mechanism entirely enclosed. 



4. No fine teeth to be damaged by over-winding. 



5. Especial value in very simple microscopes where no fine 



adjustment is fitted. 



There is much room for development along optical-bench lines, 

 and the first essential is that the main body tube and substage 



