and those of certain other Mammals. ^ 69 
stated whether these objectives were provided with the screw-collar 
adjustment for thickness of cover, but they probably were, and if so, 
doubtless all the measurements were somewhat vitiated, like others 
of the same date, by the failure to allow for the variations in magni- 
fying power produced by turning the screw-collar. Moreover, it 
must be clear that practically the fractions of a division of the eye- 
piece micrometer were only estimated, for the case in which a 
number of " equally-sized " corpuscles would be found "lying in a 
line," and just touching at their edges," without overlapping, must 
have been rare. As to the accuracy of the value assigned to the 
eye-piece micrometer, Mr. Gulliver himself says : " In the absolute 
accuracy of any micrometer applied to objects so extremely minute 
it is difficult to place implicit reliance," and he only claims " rela- 
tive exactness" for his results.* 
Turning, now, to the original essay of Welcker, we find that his 
observations give even less support than those of Gulliver to the 
notion that the blood of the dog can be distinguished from that of 
man by the microscope. Welcker 's measurements, as ordinarily 
quoted in the text-books, are '00774 of a millimeter for man, 
and • 0073 for the dog. I find, in his original paper, t that the 
mean for the dog was derived from the measurement of but ten 
corpuscles in the blood of a single terrier, the variations in this case 
being, minimum '0065 mm., maximum •0082. Now, if we turn 
to the tablet of his own measurements of human blood, we find 
that in the last measurement of the Jblood of Dr. Schweigger-Seidel, 
fifty corpuscles gave the following results : mean, • 00724 mm. : 
minimum, '0051 ; maximum, '0085, in which case the mean is a 
trifle less than that found for the dog. 
I would commend this table of Welcker's to the study of those 
who may be disposed to underrate the diversity of size which may 
be observed among the human red corpuscles ; the minimum mea- 
surement recorded in it is * 0045 mm. ; the maximum, * 0097 mm. 
The author remarks : I have always, both in animals and in man, 
found the transverse diameter of the blood-corpuscles of one and the 
same individual vary from one-fourth to one-half of the mean mea- 
surement ; and it appears that all the sizes lying between the two 
extremes are present in tolerably equal numbers, with the excep- 
tion of the smallest corpuscles, which occur for the most part singly 
and at intervals." 
I may mention further that the mean dimensions of the human 
red corpuscles so often quoted from Welcker, viz. "00774 mm., 
with a minimum of '0064 mm., and a maximum of '0086, were 
* Log. cit., p. 24. 
t H. Welcker, " Grosse, Volum uud Oberflache und Farbe der Blutkorperchen 
bei Menschen und bei Thieren," ' Zeitschrift fur Eationelle Medicin,' 3te R., 
Bd. XX. (1863), s. 237. 
J Loo. cit., p. 263. , 
