n8 



MAGNIFICATION AND MICROMETRY 



[CH. IV 



94 of the graduations to measure the diameter, the actual size of the corpuscle 

 would be 94X.08 /a = 7.52 u. 



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

 being so small, even the smallest object to be measured would require several 

 graduations to measure it. In ocular micrometers with fixed lines, small objects 

 like bacteria might not fill even one space, therefore estimations, not measure- 

 ments, must be made. For large objects, like most of the tissue elements, the 

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

 estimated is relatively small and the chance of error is correspondingly small. 



Fig. 107. Filar Ocular Micrometer with Field A (Bausch & Lomb, Optical Co.). 



§ 175. 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 cor- 

 responding change in the ocular micrometer valuation (§ 161, 171, 



176). 



\ 176. Remarks on Micrometry. — In using adjustable objectives (§24, 103), 

 the magnification of the objective varies with the position of the adjusting collar, 

 being greater when the adjustment is closed as for thick cover-glasses than when 

 open, as for thin ones. This variation in the magnification of the objective pro- 

 duces a corresponding change in the magnification of the entire microscope, and 

 the ocular micrometer valuation — therefore it is necessary to determine the mag- 

 nification and ocular micrometer valuation for each position of the adjusting 

 collar. 



While the principles of micrometry are simple, it is very difficult to get the 

 exact size of microscopic objects. This is due to the lack of perfection and uni- 



*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 ^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 f D ths millimeter of the 

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

 clude ith of T 2 oths mm. or 25th mm. ; the size of any unknown object under the 



