136 MRS, ERNEST HART. 
per 1000 should be used to ensure correct results, but so to 
graduate it as to make a solution of such a strength that, 
having previously ascertained the actual depth of the cell an 
area of ;!; mm.? multiplied by this depth shall give 1, mm.° 
In this way the necessity for arithmetical correction of each 
observation is avoided, the special adjustment of the pipette 
affording a correction which applies to all observations made 
with the instrument. Thus, taking my own Hemacytometer 
as an example, if, instead of using a 5 per 1000 solution, a 
5 per 950 solution were used, z.e. 5 parts of blood to 945 
of the diluting fluid, the result would be absolutely the same 
as if the depth of the cell were correct, or as if the error were 
corrected by calculation. Thus supposing 500 corpuscles to 
be contained in ten of the squares, 
1000 950 
500 x 10 x 190° Xe he 5,000,000. 
This device will work equally well whatever the error in 
the cell may be, if the following rule be adhered to :— 
Multiply the actual number of w in the depth of the cell 
by 5 and take the product as the number of parts of the 
solution of blood and diluting fluid to be used, the number 
of the parts of blood remaining constant at five-— 
190u x 5=950 
or, still more generally, the number of parts of blood being 
fixed, and the actual depth of the cell in » being known, 
the product of these two numbers, minus the number of 
the parts of blood, will give the necessary number of parts 
of diluting fluid required. 
With the pipette or mixer graduated according to these 
rules, it will only be necessary to add 0000 to the number 
of corpuscles counted in ten squares. 
I commend this suggestion to the notice of all who are 
using Gowers’ instrument, as its adoption will greatly 
facilitate the attainment of correct results. 
Secondly, as to the error caused by the inequality of the 
surface of the cover-glass. Any ordinary cover-glass is used 
to flatten the drop to an uniform height. Now, as every 
histologist knows, cover-glasses are rarely of an uniform 
flatness; they are generally either slightly convex or con- 
cave, hence the layer of fluid is likely to be thicker in some 
places than in others, and consequently a count made in 
one part of the cell may give very different results from 
one made in another. To remedy this defect in my instru- 
ment, I have had ground a perfectly flat cover-glass. 
Thirdly, the mode of placing the cover-glass on the cell 
