THE MECHANICAL STAGE 



175 



sensitive to focal adjustment. When, however, a critical image is 

 obtained with a f cone the conditions are changed and an objective 

 with a wide aperture becomes excessiA~ely sensitive to minute focal 

 alterations. Hence the need with the highest class of microscopic 

 investigation of at least as slow an action as can with safety to the 

 mechanism be secured, and therefore comes out the danger of 

 burdening the screw of the fine adjustment with a fraction of an 

 ounce of lifting more than can bo avoided. 



FIG. 136. Watson's new stage (1898). 



So fur as we can ascertain the speeds of the several fine adjust- 

 ments now within the reach of the worker, they are as follows, viz. : 



Speed for one revolution of the 



milled head in fraction 



of an inch 



~st = two threads to 1 mm. 

 th 



= four threads to 1 mm. 



nrr; 



th 



300 



Model 



Bausch and Lomb .... 

 Eeichert (old form) .... 



Zeiss (ordinary) 



Powell 



Baker and Swift (Campbell differential 



Eeichert new patent 



Swift vertical lever ..... 



Watson's long lever 



Zeiss's new endless screw arrangement for 



photo-micrographic stand . . . j^-th 



IV. The stage of the microscope will next call for considera- 

 tion. What is known as a mechanical stage must be a part of every 

 first-class microscope ; but by this we mean one of perfect work- 

 manship and construction, otherwise it is an impediment and not a 

 help. 



To this end we would say at the outset there must be thoroughly 

 well-made movements. The employment of levers, cams, and that 

 class of stage-gear is in practice, for critical purposes, a mere 



