MICROMETRIC NUMERATION OF THE BLOOD-CORPUSCLES. 139 
parts of the ruled space are simply divided into rectangles, 
z mm. long and + mm. wide. The cover-glass, which is 
ground accurately flat, is attached, by moistening the edges 
slightly with saliva, to a frame fixed to the sides of the slide. 
By an ingenious and delicate rack movement of this frame the 
cover glass is lowered without delay, and in a horizontal posi- 
tion down upon the drop. ‘The slide carrying the frame is 
represented in Fig. 3, 
Fie. 3. 
To make a numeration, the solution is made in Potain’s 
Mixer at the strength of 1 per 100, 200, 300, 400, or 500, as 
desired ; and, whilst being rapidly agitated, a drop is placed 
in the centre of the ruled space, and the cover-glass, having 
been previously attached to the frame, is lowered and 
clipped, so as to rest firmly on the points of the screws. To 
prevent evaporation, if desired to keep the preparation any 
length of time, a drop of water should be placed at the edge 
of the cover-glass, and allowed to run under and fill the 
vacant space between its edge and the groove. The red 
corpuscles that are lying within a group of 20 squares 
are then counted. These 20 squares, it will be remembered, 
have an area of z5 mm.*, and the depth of the fluid 
being + mm., the quantity of the solution under review will 
be g5 x 4=745 of amm.® The number of corpuscles seen, 
therefore, has to be multiplied by 100, and then again 
by the number representing the strength of the solution, and 
the product will be as before the number of corpuscles in a 
cubic millimeter of blood. 
Thus, for example, let the solution be 1 per 200, and let 
250 corpuscles be found on an area of 45 mm.?; then— 
250 x 100 x 200 = 5,000,000. 
Thus, to the number of corpuscles counted, if the solution 
