40 WORKING DISTANCE WITH THE MICROSCOPE [Cn. II 



microscope is in focus (fig. 31, 34). Strictly speaking, it is the dis- 

 tance between the objective front and the upper surface of a cover- 

 glass of the exact thickness for which the objective is corrected 



FIG. 30, 31, 32. WORKING DISTANCE AND THE COVER-GLASS. 



Slide The glass slide upon which the object is mounted. 



A Working distance with an uncovered object. 



B Working distance when a cover-glass is used and the object is in contact 

 with the cover-glass. The object represented by the solid black oblong ap- 

 pears to be elevated one third the thickness of the cover to the level Obj., where 

 it is represented by dots. 



The objective is elevated corresponding to the apparent elevation of the 

 object. 



C Working distance when a cover-glass is used and the objects are dis- 

 tributed in a stratum of Canada balsam. 



It is evident from this figure why the focus must be different for objects 

 at different depths in the balsam. 



As the working distance of an objective is practically always less 

 than its equivalent focus, one must take care to use cover-glasses thin 

 enough so that any suitable objective can be used for studying the 

 specimen. Furthermore, as microscopic specimens have considerable 

 thickness, the cover-glass should be thin enough so that the objective 

 can be lowered sufficiently to enable one to bring the lower strata 

 of the specimen in focus without bringing the objective front in con- 

 tact with the upper surface of the cover-glass (fig. 32). 



