44 DETERMINING THE THICKNESS OF THE COVER [Cn. II 



glass, then it is possible to determine how great is the combined 

 thickness of the cover-glass and layer of Canada balsam over the 

 object. 



Demonstrate the method as follows: (i) Where the object is in 

 contact with the lower surface of the cover-glass (fig. 31). Use a 

 4 mm. objective and a cover-glass y 1 /^ mm. thick. Make a black 

 ink mark on one side of the cover and a colored ink mark directly 

 opposite on the other side of the cover, or use glass pencils of two colors. 

 Set the graduations of the fine adjustment at zero (o). Place the 

 marked cover on a glass slide, and put under the micrdscope. Focus 

 with the coarse adjustment on the mark at the upper surface of the 

 cover. Then focus down with the fine adjustment until the mark 

 on the lower surface appears sharp. For verification, focus up until 

 the upper mark is again sharp. The elevation will of course be the 

 same as the lowering. If the total and partial revolutions of the 

 fine adjustment screw are noted, they will show how much the objec- 

 tive was lowered to get the lower mark in focus. In the case here 

 given it was lowered i revolution. Now as each revolution moves 

 the objective up or down o.i mm. the objective was moved down o.i 

 or T W of a millimeter. As this represents f of the thickness of the 

 cover from the effect of refraction, the whole thickness must be o.io 

 -r- = 0.15 mm. For a cover of unknown thickness with the object 

 in contact with its under surface, put an ink mark on the upper sur- 

 face of the cover and proceed exactly as above, focusing successively 

 on the object and on the ink spot. 



(2) Where the object is somewhere below the cover-glass (fig. 32). 

 In this case the thickness of the cover-glass cannot be determined, 

 but one can determine very approximately the combined thickness 

 of the cover-glass and the mounting medium over the object as fol- 

 lows: Put an ink or glass pencil mark on the upper surface of the cover- 

 glass. Focus the mark with the coarse adjustment after setting the 

 graduations of the fine adjustment at zero (o). Then focus down with 

 the fine adjustment until the object is sharp. Note the number of 

 revolutions and the partial revolution of the fine adjustment drum. 

 As this amount represents only f of the actual thickness of the 

 glass and mounting medium over the object, divide the observed 



