296 



Transactions of the Society. 



on the milled rim, a quick rotation is easily imparted to the screw, 

 which is, at the same time, under perfect control.* 



In this instrument the lower end of the steel pin is drilled and 

 countersunk, and in the recess so formed a small bicycle ball is 

 placed, which completely eliminates friction at this point. This 

 addition is by no means necessary, but is, I think, an improve- 

 ment. I had supposed that the ball would be difficult to get 

 into place, but in practice this proves not to be so. Nor has it 

 any tendency to fall out, since it is kept in position by the pressure 

 of the lever, and if the body should be accidentally raised, the 

 steel pin is not raised with it. A small perforated cap would easily 

 secure it. 



A^ 



Fig. 55. 



It will be seen that the arrangement may be adapted to any 

 instrument of similar construction, since the additional parts may, 

 as in the present case, be screwed on ; but it would, of course, be 

 better if the guide were cast in one piece with the limb. The 

 whole could then be made with no more trouble or expense than 

 the usual form. 



The turn-out device of the sub-stage condenser (figs. 54, 55) 

 needs little description. I was led to adopt it from having used a 

 somewhat similar arrangement fitted to a Petrological Microscope, 

 which proved so good and so convenient that it occurred to me 

 it might be applied with advantage to an ordinary stand. Its 

 only fault was that tlie top lens remained always flush with the 

 stage, the focusing being effected by moving the lower one. In the 

 present arrangement the whole condenser moves together. 



* As much has been said about the liability of micrometer screws to suffer 

 from wear, it may be pointed out that if the screw is made of hard silver-steel, 

 and the bush in which it works of some hard alloy, such as phosphor bronze, both 

 will wear almost indefinitely. Mild steel and brass are too often used. 



