120 



MAGNIFICATION AND MICROMETRY 



[CH. IV 



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

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

 belongs to the contiguous spaces, hence 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. 108). 



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

 enable 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 Richardson, Monthly Micr. Jour., 1874, 1875, ; Rogers, Proc. Amer. Soc. Micro- 

 scopists, 1882, p. 239; Ewell, North American Pract., 1890, pp. 97, 173. 



FIG. 108. The appearance of the coarse 



A B stage micrometer and of the fine ocular mi- 



crometer lines when using a high objective. 



(A}. The method of measuring the 

 spaces by ptdting the fine ocular mi- 

 crometer lines opposite the center of the 

 coarse stage micrometer lines. 



( B) . Method of measuring the spaces 

 of the stage micrometer by putting one 

 line of the ocular micrometer (o. m.} at the 

 inside and one at the outside of the coarse 

 stage micrometer lines (s. m.). 



FIG. 108. 



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 0.2 n 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 ($ 167, 168, 169, 175), the one given in \ 167 is impracticable, that given in 

 \ 168 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 

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

 ically the method given in \ 169 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 objects to be measured and so 

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

 the micrometer. Where the object is not exactly included by whole spaces on the 

 ocular micrometer, the chance of error comes in, in estimating just how far into a 

 space the object reaches on the side not in contact with one of the micrometer 



