MICROSCOPIC MEASUREMENTS 



21 



to find how many lines of the ocular scale are covered by the 

 object. The number of lines multiplied by the equivalent of 

 each line will be the size of the object in microns, or micro- 

 millimeters. 



MICROMETER EYE-PIECES 



Micrometer eye-pieces (Figs. 23 and 24) may be used in 

 making measurements. These eye-pieces with micrometer com- 



Fig. 24. — -Micrometer Eye-Piece 



binations are preferred by some workers, but the ocular microm- 

 eter will meet the needs of the average worker. 



MECHANICAL STAGES 



Moving objects by hand is tiresome and unsatisfactory, first, 

 because of the possibility of losing sight of the object under 

 observation, and secondly, because the field cannot be covered 

 so systematically as when a mechanical stage is used for moving 

 slides. 



The mechanical stage (Fig. 25) is fastened to the stage by 

 a screw. The slide is held by two clamps. There is a rack and 



