MICROMETRIC NUMERATION OF THE BLOOD-CORPUSCLES, 135 
ty X + = xy mm, 
The number of corpuscles observed in ten squares, there- 
fore, multiplied by fifty will give the number in a cubic 
millimeter of the solution ; and that multiplied by °° will 
give the result for a cubic millimeter of blood. 
For example, if the number of red corpuscles counted on 
ten squares is 500, the calculation is simply 
1000 
500 x 50 x reins 500 x 10,000 = 5,000,000 per mm.?, 
or, in other words, there is nothing to be done except to 
add 0000 to the number found by counting. No gradua- 
tion of the microscope is required, so that the instrument 
can be used anywhere and with any lens. This renders it 
more convenient as a clinical instrument, and it is therefore 
that which is in general use in the English hospitals. It 
gives approximately accurate clinical results. I must, how- 
ever, point out that it is liable to four serious sources of 
error, which destroy the value of observations made with it 
from an absolute and scientific point of view. These sources 
of error are—the uncertain depth of the cell; the inequality 
of the surface of the cover-glass; the method of placing the 
cover-glass on the drop; and the means used to make the 
mixture and to place the drop in the cell. Since a paper 
by two American physicians was published,! showing how 
careful observations may be vitiated by the variation in the 
depth of the cell in different instruments, the error in the 
depth has been written on the slide. In the Hemacytometer 
which I habitually use the cell has a depth of 190n, instead 
of 200u. This error necessitates a troublesome correction 
in each calculation. The correction is made by multiplying 
the number of corpuscles obtained by 20 and dividing by 
19; for let @ equal the number of corpuscles in a mm.? 
multiplied by the actual depth of the cell, 
190u:a:: 200u: z. 
This method of correction which is that recommended is, 
however, irksome when a great number of observations have 
to be made. I wish now to suggest that it may be alto- 
gether avoided by directing the instrument maker to gra- 
duate the pipette or mixer, whichever may be used, not, as 
at present, on the assumption that the depth of the cell 
accurately measures 200, and therefore that a solution of 5 
1 “ Blood-Cell Counting: a Series of Observations with the Hématimétre 
of M.M. Hayem and Nachet, and the Hemacytometer of Dr, Gowers.” 
By Drs. Henry and Naucrede.—‘ Boston Med. and Surg. Journ.,’ April, 
1879. 
