Ch. V] MEASURING WITH THE MICROSCOPE 153 



The advantage of the filar micrometer is that the valuation of one 

 graduation is so small that even the smallest object to be measured 

 would require several graduations to measure it. In ocular microm- 

 eters 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 microm- 

 eters with fixed lines answer very well, for the part which must be 

 estimated is relatively small and the chance of error is correspondingly 

 small (§ 252a). 



§ 252a. There are three ways of using the ocular micrometer, or of arriving 

 at the size of the objects measured with it: 



(1) 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 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 0.2 

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

 micrometer would include ? or 0.2 or 0.04 mm.; the size of any unknown 

 object under the microscope would be obtained by multiplying the number 

 of the divisions on the ocular micrometer required to include its image by the 

 value of one space, or in this case 0.04 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 0.04 = 0.6 mm. 



(2) By finding the number of divisions on the ocular micrometer required 

 to include the image of an entire millimeter of the stage micrometer, and usim,' 

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

 micrometer ratio. Taking the same case as in (1), suppose five divisions of 

 the ocular micrometer are required to include the image of 0.2 mm., on the 

 stage micrometer, then evidently it would require 5 4- 0.2 = 25 divisions on 

 the ocular micrometer to include a whole millimeter on the stage micrometer, 

 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 -7- 25 = -, or 0.6 mm. This method is 

 really exactly like the one in (1), for dividing by 25 is the same as multiplying 

 by 2*5 or 0.04. 



(3) 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 sizt of tin- 

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

 Suppose the stage micrometer is ruled 0.1 and 0.01 mm. and the ocular microm- 

 eter is ruled in millimeters and 0.1 mm. Taking 0.2 mm. on the stage microm- 

 eter as object, as in the other cases, suppose it requires 10 of the 0.1 mm. 

 spaces or 1 mm. to measure the real ima^e, then the real image must be magni- 

 fied 1.0-7-0.2 = 5 diameters, that is, the real image is five limes as k'reat in 

 length as the object, and the size of an object may be determined by putting 

 it under the microscope and getting the size of the real image in millimeters 



