140 MAGNIFICATION AND MICROMETRY \_CH.IV 



others considerably larger, one can nearly always manage to exactly include 

 the object by some two lines. The ocular screw micrometers (Figs. 118-120) 

 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 (\ \ 195-6) is spoken 

 of very favorably by experts who have employed it. For those especially in- 

 terested in micrometry, as in its relation to medical jurisprudence, the follow- 

 ing references are recommended. These articles consider the problem in a 

 scientific as well as a practical spirit: The papers of Prof. \Vm. 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. 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 Micrometry, Blood, and Jurisprudence, he can get on track of the 

 main work which has been and is being done. 



10 CENTIMETER RULE 



The upper edge is in millimeters, the lower in centimeters, and half 

 centimeters. 



THE METRIC SYSTEM 



UNITS. The most commonly used divisions and multiples 



,. cc-n- C r, TT,-,T> ( Centimeter (c. m. ) , i-iooth Meter; Millimeter (m.m.), i-ioooth Meter: 



t Vp \ Micron (/< ),i-ioooth Millimeter; the Micron is the unit in Micrometry(i66). 



' ' ( Kilometer, 1000 Meters; used in measuring roads and other long distances. 



THE GRAM FOR ( Milligram (in. g.), i-ioooth Gram. 

 WEIGHT . . \ Kilogram, 1000 Grams, used for ordinary masses, like groceries, etc. 



THE LITER FOR / Cubic Centimeter (c. c.), i-ioooth I,iter. This is more common than the 

 CAPACITY . [ correct form, Milliliter. 



Divisions of the L'nits are indicated by the l,atin prefixes; deci, i-ioth ; centi. t-iooth ; 

 Milli, i-ioooth ; Micro, i-i,ooo,oooth of any unit. 



Multiples are designated by Greek prefixes : deka, 10 times hecto, 100 times ; kilo, 1000 

 times ; myria 10,000 times ; Mega, 1,000,000 times any unit. 



