In- constr 



1/6 THK EISTORY AND DEVELOPMENT OF THE MICROSCOPE 



mechanical mockery. Better trust to and educate the fingers to 

 move the object tli.-ui IM- beguiled by any such practically tormenting 

 delusions. They are simply impossible as accompaniments of a 

 tirst -class micro>co| ie. 



The principle u]ion which alone a perfect mechanical stage can 

 constructed, so as to work smoothly without ' loss of time,' and en- 

 dure constant use without failure, must 

 be the employment of prism-shaped 

 plates sliding in sprung V-shaped 

 grooves, ami bearing only on four points. 

 We may test the mechanical quality 

 of the movements of a stage, as in the 

 case of the coarse adjustment, by re- 

 moving the parts, cleaning them, and 

 replacing them, when they should work 

 smoothly and without shake. Where 

 the sliding parts are tightened into 

 easilv fitting and merely ploughed 

 grooves by pressing the pinion into the 

 rack, the desirable result of smooth 

 working and instant responsiveness of 

 sliding plates to milled heads will not 

 present itself. 



l>ut besides the perfect action of the sliding parts, a perfect 

 mechanical stage should have equal speed of motion vertical!'/ ml 

 horizontally. A common fault is that the speed ot the rack work 

 giving vertical motion is greatly in excess of that of the screw giving 

 lateral or horizontal motion. If, for example, a pinion has eight 

 leaves, arid the rack it works has twenty-four teeth to the inch, then 

 three turns of the milled head (and pinion) would cause one inch of 

 movement to the stage, in order, therefore, to get the same rate of 

 movement in the lateral motion, the screw should be so pitched as 

 only to move the stage through an inch with three revolutions of the 

 "ed head. 



FIG. 137 (18!)Hj. 



in 



FIG. i:;s (1898 . 



t is most desirable thai I],, piniom l<nn!,l ! 

 \ith the movements of the stage, and the mill,-,! 



not movable 

 carrying the 

 " 



/.- as near t<> >,,,/, //,,</ /,oxx/7/A-. The best 

 Turivll's. devised iii ls::-J. where one (a, screw) is 

 other (a pinion) passes through it ; this permits both 

 al the same time \\ith one hand, giving a diagonal 



