152 Betedion of Red and White \_^7n^Z^,'^^^tl!'S 
with rare filaments of fibrin, and white blood cells imbedded in it. 
It is true that the older microscopists who rarely obtained first-rate 
definition with their lenses magnifying much beyond 500 diameters, 
were probably wise in recommending that none but the most expert 
should attempt a decision between the blood of various mammalia, 
even when fresh, for the difierence between an apparent magnitude 
of xV 9.nd y 2" of an inch may well be counted too minute to lightly 
determine a question often so momentous ; but as during the last 
three or four years opticians have furnished immersion lenses of 
2^5 and of iiich focal length, which with the highest eye-piece 
give an amplification of about 2500 and 5000 diameters respectively, 
thus rendering, with the former, the apparent size of a red disk 
from fresh human blood five-sevenths of an inch, while that of a 
corpuscle from ox blood is but half an inch across, and consequently 
little more than half the area as seen upon the stage, it seems as if 
any careful observer might now, with the aid of such objectives, be 
qualified to pronounce a positive opinion. 
It has been plausibly objected, however, as by Prof. Yirchow 
in the extract above quoted, that since the diagnosis of the different 
species of mammalian blood depends solely upon the relative size of 
the red disks, variation in the rapidity of desiccation may sometimes 
cause dried corpuscles to so deviate from the ordinary degree of 
contraction during that process as to lead the microscopist, who 
relies upon the characteristic of magnitude only, into serious or fatal 
error. In order to test the truth of this hypothesis, drops of blood 
from the finger, deposited upon pieces of muslin, were dried under 
various circumstances ; fragments of the stain removed by scraping 
were then moistened with pure water, and from each variety of 
desiccated spot, ten corpuscles selected without regard to size, as 
among those which had best retained their normal circular outline, 
were carefully measured with the micrometer. Upon comparing 
the averages of these, as appended below, it will be seen that the 
difference in the mean diameters does not amount to tt oVo o of an 
inch ; in no instance was a circular red disk observed to exhibit 
such an approximation in magnitude to those of ox blood, as could, 
by any possibility, render its difierent origin a matter of doubt. 
Table, 
Ten blood corpuscles moistened with water from a clot on muslin which had 
been dried:— Diametebs. 
Max. Min. Mean. 
In the open air at ordinary temperature . . 
Before a hot fire -^-^ 
In the afternoon sunshine st* 
In a damp, dark closet stttt 
These various experiments appearing to indicate the absence of 
any tendency in the red blood disk to undergo expansion, I was led 
Tf 8^ • ■ 3564 
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3700 •• 355 2 
