l6o THE HISTORY AND DEVELOPMENT <>F THE MICROSCOPE 



|>reci.-iou. A -nod 'coarse adjustment' or primary movable 

 part of'the instrument is of great impoi-tance. The first requisite is 

 that tin- body or movable part should move easily, smoothly, hut 

 uithoiit ' shake ' in the groove or sloi or whatever else it slides in. 

 \Ye ha\r found in practice that a har shaped like a truncated prism 

 sliding in a suitable groove acts best and longest. But a bar planed 

 true and placed in a groove ploughed to suit, it is not enough. The 



this brino-s with it a fatal 



inevitable friction determines wear, and 



Flu. 1-24. Dia-onal rack and twisted pinion devised in 1881. 



All such grooves, which are usually v-sli.-ip.-d. l,rnM be 



so thai by 'tightening up' the v's by 



s the bar or limb isagain firmly -rippi>d. Further, the 



tor its whole length along the groove but only 



fcher ( ' 1 " 1 ;l1111 ' the middle. IWell introduced these 



-' ' > 'arse adjustment ' more than 60 years 



usands of instruments in which these principles 



"" applied l.av, Keen, by sheer friction wear, soon 



'"'- since then! l!m instruments made bv 



