CH. IV] MAGNIFICATION AND MICROMETRY 135 



§ 191. Example of Measurement. — Suppose one uses 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 line- 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 94X .08/^=7.52/*. 



The advantage of the filar micrometer is that the valuation of 

 one graduation being so small, even the smallest object to be meas- 

 ured would require several graduations to measure it. In ocular 

 micrometers 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 micrometers with fixed lines answer very well, for the part 

 which must be estimated is relatively small and the chance, of error 

 is correspondingly small. 



§ 192. Obtaining the Valuation of the Combined Ocular 

 Micrometer (Fig. 120). — To obtain the valuation of this ocular 

 micrometer one proceeds exactly as for the micrometer with fixed 

 lines (§ 188), except that a partial stage micrometer space can be 

 measured by rotating the drum until the ocular micrometer exactly 

 coincides with the stage micrometer. One can then count up the 

 number of spaces on the ocular micrometer required to measure 

 one or more spaces on the stage micrometer. To this is then 

 added the 100 hundredths of a space indicated on the drum. For 

 example suppose that it required 7 complete spaces of the ocular 

 micrometer and the drum showed 50 hundredths to measure 3 

 spaces (3 hundredths mm.) on the stage micrometer, then each 

 space on the ocular micrometer would be equal to 0.03 mm. -i-7. 50= 

 0.004 mm - or 4/*- One or ^e spaces on the drum which represents 

 one hundredth of an interval on the ocular micrometer would have 

 a valuation under these conditions of only 0.04/4. This gives a 

 clear notion of the minuteness of the objects which can be measured 

 and of the smallness of the error in measuring large objects even if 

 one should get the object a few of the drum divisions too small or 

 too large. 



§ 193. Example of Measurement with the Combined 

 Ocular Micrometer. — Select an oval corpuscle of some lower 



