iS8 



MEASURING WITH THE MICROSCOPE 



[Ch. V 



lines, or from the left side of all. 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. 90, 98). 



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 dis- 

 tinctness. The necessity of using sufficient amplification in microm- 

 etry has been especially remarked upon by Richardson, Monthly 

 Micr. Jour., 1874, 1875; Rogers, Proc. Amer. Soc. Microscopists, 

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



C&rreot 



Correct 



^ncocrect 



Fig. 98. CoREECT and Incorrect Arrangement of the Ocdxar and 

 OF THE Stage Micrometer Lines. 



(From Chamot). 



The fine lines are those of the ocular micrometer and the coarse ones of the 

 stage micrometer (compare fig. 90). 



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

 the right to speak with authority expresses himseK as follows: "I 

 assume that 0.2 /x 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 (§ 247-253), the one given in § 247 is im- 

 practicable, that given in § 252-3 is open to the objection that two 

 standards are required, — the stage micrometer 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. Theo- 

 retically the method given in § 249 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 experi- 

 ence has found most safe and expeditious, and applicable to all objects, 



