Ch. V] MKASURING WITH THE MICROSCOI'I 159 



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 enclosed 

 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 lines. 

 If the ocular micrometer has some quite narrow spaces, and others 

 considerably larger, one can nearly always manage to exactly include 

 the object by some two lines. The ocular screw micrometers (fig. 91, 

 94) obviate this entirely, as the cross hair or lines traverse the object 

 or its real image, and whether this distance be great or small it can be 

 read off on the graduated wheel, and no estimation or guess work is 

 necessary. 



The new method by means of Wright's eikonometer (§ 253-254) 

 is spoken of very favorably by experts who have employed it. 



Collateral Reading for Chapter V 



Sir A. E. Wright's Principles of Microscopy. Chamot, Chemical Microscopy. 



For those especially interested in micrometry in its relation to medical 

 jurisprudence the following are recommended. They treat the subject in a 

 practical as well as in a scientific spirit. The papers of Prof. Wm. A. Rogers 

 on micrometers and micrometry, in the Amer. Quar. Micr. Jour., Vol. I. pp. 97, 

 208; Proceedings Amer. Soc. Microscopists, 1882, 1883, 1S87. Dr. M. D. Ewell, 

 Proc. Amer. Soc. Micrs., 1890; The Microscope, 1889, pp. 43-45; North Amer. 

 Pratt. 1890, pp. 97, 173. Dr. J. J. Woodward, Amer. Jour, of the Med. Sci., 

 1875. M. C. White, Article "Blood Stains," Ref. Hand-Book Med. Sciences, 

 1885. Medico-Legal Journal, Vol. XII. For the change in magnification due 

 to a change in the adjustment of adjustable objectives, see Jour. Roy. Micr. 

 Soc. 1880, p. 702; Amer. Monthly Micr. Jour., 1880, p. 67. Carpenter-Dallinger, 

 p. 270 and end of § 196. 



If one consults the medico-legal Journals; the microscopical journals, the Index 

 Medicus, and the Index Catalog of the library of the Surgeon General's Office, 

 under Micometry, Blood, and Jurisprudence, he can get on track of the main 

 work which has been and is being done. 



