45 MA GNIFICA TION AND DRA WING. 



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 give 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 (§§ no, in, 116), so that if the meth- 

 ods are correct and the work carefully enough done the same results 

 should be obtained by each method. See general remarks on microm- 

 etry' (§117).* 



* 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 fre- 

 quently employed. Thus, suppose with a given optical combination and tube- 

 length it required five divisions on the ocular micrometer to include the im- 

 age of T 2 ,jths millimeter of the stage micrometer, then obviously one space on the 

 ocular micrometer would include Ath of T 2 oths mm. or 5 Vth 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, ^yth 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 3-5 = ftlis, or 0.6 mm. 



(B) By finding the number of divisions on the ocular micrometer required to in- 

 clude 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 divisions of the ocular microm- 

 eter are required to include the image of y 2 7 ths mm., on the stage micrometer, then 

 evidently it would require 5 -f- y 2 ^ = 25 divisions on the ocular micrometer to in- 

 clude a whole millimeter on the stage micrometer, then 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, sup- 

 pose 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 = f or 

 0.6 mm. This method is really exactly like the one in (A), for dividing by 25 is 

 the same as multiplying by 2 Vth. 



(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 em- 

 ploying this method a stage micrometer is used as object and the size of the image 

 of one or more divisions is measured by the ocular micrometer, thus : Suppose 

 the stage micrometer is ruled in y (7 th and yjjrjth tarn, and the ocular micrometer is 

 ruled in millimeters and y^th mm. Taking T 2 n th mm. on the stage micrometer as 

 object, as in the other cases, suppose it requires 10 of the y^th mm. spaces or 

 1 mm. to measure the real image, then the real image must be magnified TT } -f- 

 y% = 5 diameters, that is the real image is five times as great in length as the ob- 

 ject, and the size of an object may be determined by putting it under the micro- 



