138 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 yjjj 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 ij revolutions, or 125 graduations, on the wheel, to 

 measure the T J 7 mm., or lo/x (157), then one of the graduations on 

 the wheel would measure lo/x divided by I25 = .o8/x. 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 .08 n = J. 52^. 



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. 



