134: MRS. ERNEST HART. 
instrument in the pure air of Paris, I am unable in 
London, where nothing is clean, not even distilled water, 
to keep the tube quite free from dirt. 
Hayem’s method differs altogether from that of Malassez. 
The unit is arrived at by means of a cell + mm. deep, of 
which an area of 3 of a square mm. is marked off. This 
gives us, therefore, 1 x 55 = +4, of a mm’. The area of 
=!5 square mm. (which is, of course, =; of 1,000,000 square 1) 
is obtained by using an ocular micrometer, on which is drawn 
an oblong, 5 mm. long by 4 mm. wide, and divided into 20 
squares. By means of a stage micrometer the microscope 
is graduated so that the 5 mm. exactly correspond to an ob- 
jective length of 250u. The mixture of blood and the pre- 
serving fluid is made at a strength of 8 per 1000. The 
method of mixing is that invented by Vierordt. A pipette, 
holding 8 cubic mm., is used to measure the blood; another, 
of acalibre of 992 cubic mm., to measure the preserving 
solution. The two fluids are mixed in an open glass vessel 
by means of a glass rod. The same rod is also used to 
deposit the drop on the slide. The cover-glass is kept i 
situ by the capillary attraction existing between two moist 
glass surfaces, a drop of water or saliva being placed at the 
edge of the cover-glass, and allowed to run underit. The 
chief objections to this instrument are the uncertain depth 
of the cell, the clumsy method of mixing, the possible ele- 
vation of the cover-glass by allowing too much water to run 
under, it and also the same objection made to Malassez’s capil- 
lary Compte-Gilobules just considered, namely, that, owing 
to the necessity of graduating the microscope, it is of limited 
use as a clinical instrument. 
In Gowers’ Hemacytometer,! which is a modification of 
Hayem’s, a very decided improvement is made. In the 
depth of the cell and in the old-fashioned mode of mixing, 
it is identical with that of Hayem; the solution of blood 
used being, however, at 5 per 1000 instead of at 8 per 1000. 
The improvement consists in measuring the area and drawing 
the squares in which the corpuscles are to be counted upon 
the floor of the cell itself. Squares, with sides , of a mm. 
long, are drawn on the floor of the cell. The area of each, 
therefore, is +4, of asq. mm. The cell having a depth of 
4+ mm., and any 10 squares an area of +; of a sq. mm., the 
cubic contents of any ten squares taken within the cell 
will be— 
1 “On the Numeration of the Blood-corpuscles,” by Dr. Gowers, 
‘ Lancet,’ Dec., 1877. 
