108 MAGNIFICATION AND MICROMETRY. [CH. IV. 



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 consid- 

 erably larger, one can nearly always manage to exactly include the object by some 

 two lines. The ocular screw-micrometer (Fig. ioo) obviates this entirely as the 

 cross hairs 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. 



For those especially interested in micrometry, as in its relation to medical juris- 

 prudence, the following references are recommended. These articles consider the 

 problem in a scientific as well as a practical 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, 1887. Dr. M. D. 

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

 Amer. Pract, 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, 1S80, p. 702 ; 

 Amer. Monthly Micr. Jour., 18S0, p. 67. 



If one consults the medico-legal journals, the Index Medicus, and the Index 

 Catalog of the Library of the Surgeon General's Office, under Micrometry, Blood, 

 and Jurisprudence, he can get on track of the main work which has been and is be- 

 ing done. 



