146 LABORATORY BACTERIOLOGY 
Note the position of the graduated wheel and of the teeth of the 
recording comb, and then rotate the wheel until the movable lines 
traverse one space on the stage micrometer. Each tooth of the 
recording comb indicates a total revolution of the wheel, and by 
noting the number of teeth required and the graduations on the 
wheel, the revolutions and parts of revolution required to measure 
the z45 mm. of the stage micrometer can be easily noted. Measure 
in like manner four or five spaces and get the average. Suppose 
this average is 1} revolutions, or 125 graduations, on the wheel, to 
measure the 745 mm., or lou (157), then one of the graduations on 
the wheel would measure toy divided by 125=.084. In using this 
valuation for actual measurement, the tube of the microscope and 
the objective must be exactly as when obtaining the valuation (165). 
Example of Measurement. — Supposing one used the red blood 
corpuscles of a dog, or monkey, etc., every condition being as when 
the valuation was determined, one notes very accurately how many 
of the graduations on the wheel are required to make the movable 
lines traverse the object from edge to edge. Suppose it requires 94 
of the graduations to measure the diameter, the actual size of the 
corpuscle would be 94 x .o8¢=7.52y. 
The advantage of the filar micrometer is that the valuation of one 
graduation being so small, even the smallest object to be measured 
would require several graduations to measure it. In ocular microm- 
eters with fixed lines small objects like bacteria might not fill even 
one space; therefore estimations, not measurements, must be made. 
For large objects, like most of the tissue elements, the ocular mi- 
crometers with fixed lines answer very well, for the part which must 
be estimated is relatively small, and the chance of error is corre- 
spondingly small. 
Obtaining the Ocular Micrometer Valuation for an Ocular Microm: 
eter with Fixed Lines (Figs. 33, 34, p. 25).— Use the stage microm- 
eter as object. Light the field well and look into the microscope. 
The lines of the ocular micrometer should be very sharply defined. 
If they are not, raise or lower the eye-lens to make them so, that is, 
focus as with the simple magnifier. 
When the lines of the ocular micrometer are distinct, focus the 
microscope (45, 46, 56) for the stage micrometer. The image of 
the stage micrometer will appear to be directly under or upon the 
ocular micrometer. 
