70 On^ the Similarity hetween the Bed Blood-corpuscles of Man 
not derived from the wliole of this table, but from four sets of 
measurements of his own blood only, of which two were from dry 
preparations and two from the moist blood. He tells us that he 
selected the mean '00774 mm. because it was derived from his 
own blood, which he had used in a previous research on the number 
of blood-corpuscles, and thought best, therefore, to use also in the 
computation of their volume, which is one of the chief subjects 
discussed in his paper. The mean of eight other measurements 
from five different individuals was '00768 mm. The blood of 
a chlorotic woman gave '00656 mm. as the mean of the cor- 
puscles examined moist, and '00693 mm. as their mean when 
examined dry. 
Welcker made his measurements with Kellner's System III., 
ocular II., magnifying about 620 diameters, and by a dehcately ruled 
eye-piece micrometer, each division of which, with the power used, 
had a value of ' 001723 mm., as determined by the stage-micro- 
meter. A human blood-corpuscle fell within four or five of these 
divisions, while, on account of the great delicacy of the ruling, 
fifths or even tenths of a division could be estimated with tolerable 
exactness. The stage-micrometer itself was a millimeter in 
one hundred parts ruled by Lerebours, and which Welcker had 
verified by comparison with a standard scale, in a manner which 
he describes in full, and which is worthy of study. He measured, 
as a rule, 50 blood-corpuscles from each sample, and these were 
not selected, but taken indiscriminately one after the other, as they 
came under the scale while the specimen was being moved along. 
Other observers besides G-ulliver and Welcker have recorded 
minute differences in the average size of the red corpuscles of man 
and the dog. Thus, Carl Schmidt* estimates the average dia- 
meter for man at ' 0077 mm. — for the dog at ' 0070 mm. A. 
KoUikert fixes the mean for man at '0033 of a Paris line 
(= -0751 mm.) — that of the dog at '0031 of a Paris line 
(= -00709 mm.). On the other hand, EriedbergJ makes the 
blood-corpuscles of the dog the largest, stating that he finds the 
human corpuscles measure from '0070 to '0058 mm. — those of 
the dog from '0054 to '0080. 
For myself, after repeated measurements of the blood of the 
dog and of human blood, I can only say that I find no constant 
difference between them, whether the fresh blood or thin layers dried 
on glass be selected for measurement. The mean of fifty corpuscles 
taken at hazard is seldom twice the same, and sometimes that of 
human blood, sometimes that of dog's blood, is a trifle the largest. 
The following measurements, intended to illustrate these facts, 
* Op. cit. 
t ' A Manual of Human Microscopic Anatomy,' London, 1860 ; pp. 519 and 525. 
X Op. cit. 
