CH. IV.~\ MAGNIFICATION AND MICROMETRY. 105 



100 spaces on the ocular micrometer to include 1 millimeter on the stage 

 micrometer, then as before 1 space of the ocular micrometer would have 

 a valuation of y^-jjth millimeter for the purposes of micrometry ; and the 

 size of any minute object may be determined by multiplying this valua- 

 tion 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 x^-th mm. x 8 = T | ¥ mm. or 80 /u. (§ 157). 



§ 165. Varying the Ocular Micrometer Valuation. — Any change 

 in the objective, the ocular or the tube-length of the microscope, that 

 is to say, any change in the size of the real image, produces a corre- 

 sponding change in the ocular micrometer valuation (§ 152, 161). 



§ 166. Micrometry with the Ocular Micrometer. — Use the 3 mm. 

 (;Ms in. ) objective and 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 (§ 159, 160). 



Count the divisions on the ocular micrometer required to enclose or 

 measure the long and the short axis of each of the three corpuscles, 

 then multiply the number of spaces in each case by the valuation of the 

 ocular micrometer for this objective, tube-length and ocular, and the 

 results will represent the actual length of the axes of the corpuscles in 

 each case. 



The same corpuscle is, of course, of the same actual size, when meas- 

 ured in each of the three ways, so that if the methods are correct and 

 the work carefully enough done, the same results should be obtained by 

 each method. See general remarks on micrometry (§ 167).* 



* There are three ways of using the ocular micrometer, or of arriving at the size 

 of the objects measured with it : 



(A) By finding the value of a division of the ocular micrometer for each optical 

 combination and tube-length used, and employing this valuation as a multiplier. 

 This is the method given in the text, and is the one most frequently employed. 

 Thus, suppose with a given optical combination and tube-length it required five 

 divisions on the ocular micrometer to include the image of T 2 jths millimeter of the 

 stage micrometer, then obviously one space on the ocular micrometer would in- 

 clude ^th of ^ths mm. or ^ z lh mm. ; and the size of any unknown object under 

 the microscope would be obtained by multiplying the number of divisions on the 

 ocular micrometer required to include its image by the value of one space, or in 

 this case, j'jth mm. Suppose some object, as the fly's wing, required 15 spaces of 

 the ocular micrometer to include some part of it, then the actual size of this part 

 of the wing would be 15 X 15 = f ths, or 0.6 mm. 



