THE SUB-STAGE 



189 



has 40 threads to the inch, and works through an immovable fitting, 

 the thread is discontinued at C, and from C to D a screw having 50 

 threads to the inch is cut, working through a fitting E. If now 

 the milled head F be rotated 40 times, the screw A C will have 

 travelled one inch. So will the screw C D as it is cut on the same 

 stem, but it would take 50 revolutions of screw C D to travel one 

 inch through the fitting E, hence the fitting E must have been 

 carried up bodily the remaining 10 revolutions that is to say, ith 



FIG. 153. Baker's fine adjustment to sub-stage (1888). 



of an 



of an inch therefore one revolution raises the fitting E 

 inch. 



The fitting E is attached to the sub-stage G through a slot cut in 

 the cover of the adjustment ; the cover is also grooved on either side 

 to receive that part of the sub-stage H which insures the true 

 vertical movement so essential with this screw. 



It is almost a matter of compulsion to refer here to a com- 

 paratively recent arrangement known as a swinging sub-stage, which 

 is, as its name implies, a sub-stage so arranged as to be capable of 



