1894.] MICROSCOPICAL JOURNAL. 15 



Measurement of Blood Corpuscles. 



Bv M. D. EWELL, M. D. 

 CHICAGO, ILL. 



[Republished by request from the American Monthly Microscopical Jour- 

 nal for August 1885, pp. 150-1.] 



For some time past I have been endeavoring, for m y 

 own satisfaction, to determine whether there is a con- 

 stant average size of the human red blood corpuscles, 

 with the view ultimately to determine whether it is pos- 

 sible, by means of micrometric measurements, to distin- 

 guish human blood from tlie blood of domestic animals. 



In order that the results arrived at may be compared 

 with those of other observers, I think it proper to state 

 at the outset the methods and instruments employed. 



The first requisite is obviously a correct standard of 

 length, and the accurate determination of the value of 

 the eye-piece micrometer used. This preliminary work 

 has engaged much of my time and attention for several 

 months past, and I have finally succeeded in obtaining 

 two very accurate standards. The one of these which 

 has been used as the standard of the measurements here- 

 inafter given consists of lines ruled by Prof. W. A. Rog- 

 ers, of Cambridge, Massachusetts (who is recognized as 

 the highest authority upon questions of this sort), upon 

 speculum metal at intervals of 1-2000 inch. The relative 

 and absolute corrections of this standard have been de- 

 termined by Prof. Rogers with very great accuracy, and 

 the value of a division of the eye-piece micrometer de- 

 scribed below was determined by taking an arithmetical 

 mean of a long series of measurements of different inter- 

 vals of 1-2000 inch, so as to eliminate as nearly as possi- 

 ble all errors of graduation and of measurement, and the 

 A^alue of one division of the micrometer was thus found 

 to be .0000009925 or, approximately, 1-lOOOOCO inch. The 

 stand used, with mechanical stage and Abbe condenser 



