l6o THE HISTORY AND DEVELOPMENT OF THE MICROSCOPE 



and precision. A good 'coarse adjustment' or primary movable 

 part of the instrument is of great importance. The first requisite is 

 that the body or movable part should move easily, smoothly, but 

 without * shake ' in the groove or slot or whatever else it slides in. 

 We have found in practice that a bar 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 

 inevitable friction determines wear, and this brings with it a fatal 



PIG. 124. Diagonal rack and twisted pinion devised in 1881. 



* shake.' All such grooves, which are usually Y-shaped, should be 

 cut and sprung on one side, so that by ' tightening up ' the v's by 

 means of screws the bar or limb is again firmly gripped. Further, the 

 bar should not ' bear ' for its whole length along the groove, but only 

 on points at either end and in the middle. Powell introduced these 

 prime essentials to a good 'coarse adjustment ' more than 60 years 

 ago ; yet what thousands of instruments in which these principles 

 have not been applied have been, by sheer friction wear, soon 

 changed into useless brass since then ! But instruments made by 



