172 THE HISTORY AND DEVELOPMENT OF THE MICROSCOPE 



levers, h, h, which in turn press the arched piece with its appendix f 

 on to the prism support. The principal screw has three threads 

 to the millemeter, which by the levers is reduced by about one 

 third. The pointer for reading the micrometer scale on the 

 milled head is conveniently arranged so that it can be changed to any 

 figure on the scale. The speed of the adjustment is ^y^th inch to 

 one revolution of the milled head. 



We may now profitably consider the best forms of fine adjust- 

 ment that apply to the Lister model, and one of the steadiest and 



A 



FIG. 184. Nelson's model with Swift's fine- 

 adjustment screw to the left hand (1882). 



FIG. 185 (1885). 



most delicate of these is that devised by Messrs. Watson and Sons. 

 The entire body is raised or lowered by means of a milled head fixed 

 to a screw having a hardened steel point, acting on a lever with 

 hardened and highly polished contact surfaces, against a point 

 attached to the body-slide, in a perfect dovetailed fitting, about 

 2^ inches long. This is seen in the section shown in fig. 132. By 

 turning the milled head the hard steel lever B, which has its fulcrum 



