CH. IV} 



MAGNIFICATION AND MICROMETRY 



139 



from the inside of one bordering 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. 123). 



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 es- 

 pecially remarked upon by Richardson, Monthly Micr. Jour., 1874, 1875,; 

 Rogers, Proc. Amer. Soc. Microscopists, 1882, p. 239; Ewell, North American 

 Pract., 1890, pp. 97, 173. 



B 



Fig. 123. The appearance of the 

 coarse stage micrometer 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 course stage micrometer lines. 



(B). Method of measuring the 

 spaces of the stage micrometer by 

 one line of the ocular micrometer 

 {0. m. ) at the inside and one at the 

 outside of the course stage microm- 

 eter lines (s. m.). 



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

 right to speak with authority expresses himself as follows: "/ assume that 

 0.2H is the limit of precision in microscopic measures beyond which it is im- 

 possible to go with certainty." W. A. RogersProc. Amer. Soc. Micrs., 1883, p. 

 198. 



In comparing the methods of micrometry with the compound microscope 

 given above (§ 183, 184, 185, 189, 191, 193, 196,), the one given in \ 183 is 

 impracticable, that given in (j 184 is open to the objection that two standards 

 are required, — the stage micrometer, and the steel rule; it is open to the fur- 

 ther objection that several different operations are necessary, each operation 

 adding to the probability of error. Theoretically the method given in \ 185 is 

 good, but it is open to the very serious objection in practice that it requires so 

 many operations which are especially liable to introduce 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 mi- 

 crometer lines. If the ocular micrometer has some quite narrow spaces, and 



