MICROMETRIC NUMERATION OF THE BLOOD-CORPUSCLES, 137 
is faulty; whether it is dropped on horizontally or laid on 
gently at an inclined plane, the uniform diffusion of the 
corpuscles through the fluid is disturbed. 
Fourthly,in the method of mixing and placing the drop on 
the cell, errors are caused by the white corpuscles adhering 
to the sides of the vessels used for mixing, and by evapo- 
ration from the little open cup in which the solution is 
kept. Further, in placing the drop on the slide, unless the 
manceuvre is very quickly executed, the red corpuscles 
gravitate tothe bottom of the drop, and are thus deposited 
and form a thicker collection in the centre of the drop than 
at the periphery. The white corpuscles also, by adhering 
to the glass rod, introduce a source of error in estimating 
the right proportion between white and red corpuscles. 
It is, I think, to be regretted that, in introducing this really 
useful clinical instrument, Dr. Gowers should have adopted 
the old, clumsy, and discarded method of making the solu- 
tion, instead of using Potain’s Mixer, the use and value of 
which were already known. By this mixer a solution of 
blood at 100, 200, 300, 400, or 500, as desired, is made in 
a closed vessel, evaporation thus being prevented ; the drop 
is deposited on the slide whilst the corpuscles are in rapid mo- 
tion and before they have had time to gravitate to the bottom 
of the drop. For the last eighteen months I have, when 
using Gowers’ Hemacytometer, substituted Potain’s Mixer 
in the place of the apparatus provided, and with the result 
of obtaining much more uniform counts in different parts of 
the cell, whereas previously the want of uniformity was often 
very marked. 
By the means I have indicated, namely, by correcting the 
error in the depth of the cell, by substituting a perfectly flat 
cover-glass for one that may or may not be flat, and by using 
Potain’s Mixer for making the solution, a useful and nearly 
accurate clinical instrument can be made of Gowers’ Hema- 
cytometer. As it isat present arranged, the results obtained 
by it are often misleading, unless the mean of a great 
number of counts be taken. Single observations are likely 
to lead to the most fallacious conclusions, and are not at 
all trustworthy, whether for scientific or clinical data. 
In Malassez’s new Compte-Globules he has adopted 
the great improvement introduced by Gowers, of drawing 
the squares on the surface of the slide. He has more- 
over succeeded, by many ingenious contrivances in care- 
fully avoiding all the sources of error in Hayem’s and Gowers’ 
instruments above enumerated, to several of which I had 
occasion to call his attention. This new MICROMETRIC GRA- 
