The Microscope. 183 



SELECTIONS. 



MEASUREMENT OF BLOOD CORPUSCLES. 



For some time past I have been endeavoring, for my own 

 satisfaction, to determine whether there is a constant average 

 size of the human red blood-corpuscles, with the view ultimately 

 to determine whether it is possible,by means of micrometric meas- 

 urements, to distinguish human blood from the 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 fully 

 succeeded in obtaining two very accurate standards. The one 

 of these which has been used as the standard of the measur- 

 ments hereinafter given, consists of lines ruled by Prof. W. A. 

 Rogers, of Cambridge, Massachusetts, (who is recognized as the 

 highest authority upon questions of this sort) upon speculum- 

 metal at intervals of 1-2,000 inch. The relative and absolute 

 corrections of this standard have been determined by Prof. 

 Rogers with very great accuracy, and the value of a division of 

 the eye-piece micrometer described below was determined by 

 taking an arithmatical mean of a long series of measurments 

 of different intervals of 1-2,000 inch, so as to eliminate as nearly 

 as possible, all errors of graduation and of measurement, and 

 the value of one division of the micrometer was thus found to 

 be .000,000,992,5 or approximately 1-1,000,000 inch. The stand 

 used with mechanical stage and Abbe condenser, was made by 

 Mr. Walter H. Bullock of this city, and is of the pattern styled 

 by him the •''Biological Stand." 



The actual tube length was 8.91 inches from end of nose- 

 piece to upper end of draw- tube. 



The cob-web eye-piece micrometer used was also made by 

 Mr. Bullock, the pitch of the screw being | millimeter, and the 



