i 3 2 MAGNIFICATION AND MICROMETRY [CH. IV 



puscles, multiply the number of spaces in both cases 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 

 measured in each of the three ways, so that if the methods are cor- 

 rect and the work carefully enough done, the same results should 

 be obtained by each method. (§ 197.)* 



* 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 i9 the method given in the text, and 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 , 2 ;, 

 millimeter of the stage micrometer, then obviously one space on the ocular 

 micrometer would include \ of ^ mm. or ^ mm. ; 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, 5 ' 5 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 15X^=1. or 0.6 mm. 



(B) By finding the number of divisions on the ocular micrometer re- 

 quired to include the image of an entire millimeter of the stage micrometer, 

 and using this number as a divisor. This number is also sometimes called the 

 ocular micrometer ratio . Taking the same case as in (A), suppose five divi- 

 sions of the ocular micrometer are required to include the image of ,% mm., 

 on the stage micrometer, then evidently it would require 5-=- r 2 5 =25 divisions 

 on the ocular micrometer to include a whole millimeter on the stage microme- 

 ter, and the number of divisions of the ocular micrometer required to measure 

 an object divided by 25 would give the actual size of the object in millimeters 

 or in a fraction of a millimeter. Thus, suppose it required 15 divisions of the 

 ocular micrometer to include the image of some part of the fly's wing, the 

 actual size of the part included would be 15-^25=! or 0.6 mm. This method 

 is really exactly like the one in (A), for dividing by 25 is the same as multi- 

 plying by sV- 



(C) By having the ocular micrometer ruled in millimeters and divisions 

 of a millimeter, and then getting the size of the real image in millimeters. 

 In employing this method a stage micrometer is used as object and the size of 

 the image of one or more divisions is measnred by the ocular micrometer, 

 thus : Suppose the stage micrometer is ruled -fa and T J 7 mm. and the ocular 

 micrometer is ruled in millimeters and fa mm. Taking T % mm. on the stage 

 micrometer as object, as in the other cases, suppose it requires 10 of the T \ 

 mm. spaces or 1 mm. to measure the real image, then the real image 



