CH. IV] MAGNIFICATION AND MICROMETRY 131 



ocular or changing the position of the stage micrometer or both if 

 necessary, and then make any two lines of the stage micrometer 

 coincide with any two on the ocular micrometer. To do this it may 

 be necessary to pull out the draw-tube a greater or less distance. 

 See how many spaces are included in each of the micrometers. 



Divide the value of the included space or spaces on the stage 

 micrometer by the number of divisions on the ocular micrometer 

 required to include them, and the quotient so obtained will give the < 

 valuation of the ocular micrometer in fractions of the unit of 

 measure of the stage micrometer. For example, suppose the milli- 

 meter is taken as the unit for the stage micrometer and this unit is 

 divided into spaces of T J T and T ^ ¥ millimeters. If with a given optical 

 combination and tube-length it requires 10 spaces on the ocular mi- 

 crometer to include the real image of T V millimeter on the stage mi- 

 crometer, obviously one space on the ocular micrometer includes 

 only one-tenth as much, or j 1 ^ mm.-MO^^ mm. That is, each space 

 on the ocular micrometer includes T ^ of a millimeter on the stage 

 micrometer, or T ^ T millimeter of the length of any object under the 

 microscope, the conditions remaining the same. Or, in other words, 

 it requires 100 spaces on the ocular micrometer to include 1 milli- 

 meter on the stage micrometer, then as before, 1 space of the ocular 

 micrometer would have a valuation of -j-J-^ millimeter for the pur- 

 poses of micrometry. The size of any minute object may be deter- 

 mined by multiplying this valuation of one space by the number of 

 spaces required to include it. For example, suppose the fly's wing 

 or some part of it covered 8 spaces on the ocular micrometer, it 

 would be known that the real size of the part measured is T ^ T mm. 

 x8= T |finm. or 80 /t (§ 182). See Mark, Jour. Applied Micro- 

 scopy, Vol. I, p. 4. 



§ 189. Micrometry with the Ocular Micrometer. — Use the 

 3 mm. (_^in.) objective with the preparation of Necturus blood- 

 corpuscles as object. Make certain that the tube of the microscope 

 is of the same length as when determining the ocular micrometer 

 valuation. In a word, be sure that all the conditions are exactly as 

 when the valuation was determined, then put the preparation under 

 the microscope and find the same three red corpuscles that were 

 measured in the other ways (§ 184-185). 



Count the divisions on the ocular micrometer required to enclose 

 or measure the long and the short axis of each of the three cor- 



