176 THE HISTORY AND DEVELOPMENT OF THE MICROSCOPE 



mechanical mockery. Better trust to and educate the fingers to 

 move the object than be beguiled by any such practically tormenting 

 delusions. They are simply impossible as accompaniments of a 

 first-class microscope. 



The principle upon which alone a perfect mechanical stage can 

 be 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, and 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 \\ork 

 smoothly and without shake. Where 

 the sliding parts are tightened into 

 easily fitting and merely ploughed 

 grooves by pressing the pinion into the 

 rack, the desirable result of smooth 

 working and instant responsiveno- of 

 sliding plates to milled heads will not 

 present itself. 



But besides the perfect action of the sliding parts, a perfect 

 mechanical stage should have equal speed of motion vertically and 

 horizontally. A common fault is that the speed ot the rackwork 

 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, and 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 

 milled head. 



FIG. 137 (1898). 



C C F F 



FIG. 138 (1898). 



It is most desirable that the pinions should be fixed, not movable 

 with the movements of the stage, and the milled heads carrying the 

 respective parts should be as near to each other as possible. The best 

 form is that of Turrell's, devised in 1832, where one (a screw) is 

 hollow, and the other (a pinion) passes through it ; this permits both 

 to be turned at the same time with one hand, giving a diagonal 



