CH. IV.] 



MAGNIFICATION AND MICROMETRY. 



107 



formity of micrometers, and the difficulty of determining the exact limits of the 

 object to be measured. Hence, all microscopic measurements are only approxi- 

 mately correct, the error lessening with the increasing perfection of the apparatus 

 and the skill of the observer. 



A difficulty when one is using high powers is the width of the lines of the mi- 

 crometer. If the micrometer is perfectly accurate half the width of each line be- 

 longs to the contiguous spaces, heuce one should measure the image of the space 

 from the centers of the lines bordering the space, or as this is somewhat difficult 

 in using the ocular micrometer, one may measure from the inside of one border- 

 ing line and from the outside of the other. If the lines are of equal width this is 

 as accurate as measuring from the center of the lines. Evidently it would not be 

 right to measure from either the inside or the outside of both lines (Fig. 101). 



It is also necessary in micrometry to use an objective of sufficient power to en- 

 able one to see all the details of an object with great distinctness. The necessity 

 of using sufficient amplification in micrometry has been especially remarked upon 

 by Richardsou, Monthly Micr. Jour., 1874, 1875 ; Rogers, Proc. Amer. Soc. Micro- 

 scopists, 1S82, p. 239; Ewell, North American Pract., 1890, pp. "97, 173. 



Fig. 10:. The appearance of the coarse 

 stage and of the fine ocular micrometer 

 lines when using a high objective. 



(A). The method of measuring the spaces 

 by putting the fine ocular micrometer lines 

 opposite the center of the coarse stage mi- 

 crometer lines. 



(B). Method of measuring the spaces of 

 the stage micrometer by putting one line 

 of the ocular micrometer (o.m.) at the in- 

 side and one at the outside of the coarse 

 stage micrometer lines (s.m. ). 



B 



Fig. ioi. 



As to the limit of accuracy in micrometry, one who has justly earned the right 

 to speak with authority, expresses himself as follows : " I assume that 0.21.1 is the 

 limit of precision in microscopic measures, beyond which it is impossible to go 

 with certainty." W. A. Rogers, Proc. Amer. Soc Micrs., 1883, p. 198. 



In comparing the methods of micrometry with the compound microscope, given 

 above (158, 159, 160, 166), the one given in g 158 is impracticable, that given in 

 $ 159 is open to the objection that two standards are required, — the stage microme- 

 ter, and the steel rule ; it is open to the further objection that several different ope- 

 rations are necessary, each operation adding to the probability of error. Theoret- 

 ically the method given in \ 160 is good, but it is open to the very serious objection 

 in practice that it requires so many operations which are especially liable to intro- 

 duce errors. The method that experience has found most safe and expeditious, 

 and applicable to all objects, is the method with the ocular micrometer. If the 

 valuation of the ocular micrometer has been accurately determined, then the only 

 difficulty is in deciding on the exact limits of the object to be measured and so ar- 

 ranging the ocular micrometer that these limits are inclosed by some divisions of 



