USING THE MICROSCOPE * 
315 
preparation on the stage, using the same objective and ocular micro¬ 
meter, and note how many spaces a pollen grain covers. If the pollen 
grain covers five spaces, its diameter is five times 15.3 or 76.5 /i. 
In the same way, the value of a space in the ocular when used with 
the other objectives should be determined. The values for three 
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Fig. 117.—Card scale for practical use 
or four objectives may be written upon an ordinary slide label and 
pasted upon the base of the microscope for convenient reference. 
This method is the best one for measuring spores and for most 
measurements in taxonomy. 
Measuring by Means of Camera Lucida Sketches. —This method 
is of great importance in research work, because various details can 
