1 88 THE HISTORY AND DEVELOPMENT OF THE MICROSCOPE 



and from the front in fig. 151. This is brought round at one end 

 at right angles to the front. The fulcrum of this lever is at C, and 

 it fits under the pin D which is attached to a dovetailed piece, having 

 at the back of it enclosed in a metal casing the counteracting spring 



FIG. 150. FIG. 151. 



Watson's sub-stage fine adjustment (1899). 



shown in fig. 151 ; when, therefore, the lever is depressed at B, the 

 sub-stage is raised at D and vice versa. The milled head A is placed 

 at the side of the stage of the microscope towards the back slightly 



higher than the surface of the 

 stage. 



The fine sub-stage adjust- 

 ment of these makers as applied 

 to their ' Royal ' microscope is 

 shown as it is in its complete 

 form in fig. 152. 



Another sub-stage fine ad- 

 justment has been devised by 

 Baker, which, we are of opinion, 

 it will be of advantage to the 

 student to understand. It em- 

 ploys the differential screw, and 

 by this means obtains a very 



FIG. 152. Sub-stage fine adjustment com- slow movement. The student has 

 plete in ' Royal ' microscope (1899). already understood that the prin- 

 ciple of this screw is the cutting 



of two threads of a different ' pitch,' one at either end of the screw, 

 the proportion of one to the other determining the amount of move- 

 ment. The threads found most suitable for their sub-stage fine ad- 

 justment were 40 and 50 to the inch. In fig. 153 the screw A C 



