269 
and then quickly discharged upon the glass-slide in one or several 
measured droplets, first samples being rejected. The dehaemoglo- 
binized thick film is fixed before staining in order to avoid loss, 
and the whole area is scrupulously searched with the aid of a sliding 
stage. 
Mr. M. Greenwood, Junr., informs me that if n is the number 
of parasites counted in the unit (say 1 c.mm.) of blood, and m is 
the number of such units in the patient’s whole body, then the total 
number of parasites in him, assuming uniform distribution, will 
be mi ± Q'6j449M^/n. Thus the probable percentage error is 
67'449/y/w, the factor m cancelling out from the ratio. This is 
important, because it shows that the error depends, not on the 
magnitude of the measured sample taken, but upon the number of 
parasites actually counted in it. Hence, in order to obtain results 
within an assigned error, we take a large sample when the parasites 
are scarce and a small one when they are numerous; the number of 
parasites which we must find and count being given by the formula 
n = 4550/d 2 , where e is the permissible error. 
The greatest error occurs when the parasites are so scarce that 
it is difficult to find enough of them to count up to the required 
standard; or when they are so numerous that it is difficult to 
measure a volume of blood small enough to contain an easil> 
countable number of them. In these preliminary researches we have 
seldom examined at one sitting more than 1 c.mm. of blood, or less 
than £ c.mm. 
When the parasites were very numerous we have sometimes fallen 
back on estimating their number by comparison with leucocytes, but 
are now elaborating improvements and special instruments for 
such, and other, details. The method is also accurate and rapid 
for other blood parasites, and for leucocytes. Other sources of 
error are (1) inaccurate measurements of the quantity of blood used, 
and (2) difficulty of seeing or distinguishing badly stained parasites. 
One kilogram of blood of average density (1057 5) contains 
945-626 cubic millimetres, and the blood in a mans bod> is 
estimated to weigh about 49 per cent, of the total body weight, 
from which data it is easy to compute the total number of parasites 
in a patient. A man of 6474 kilograms, or about ten stone, will 
contain about 3,000,000 c.mm. of blood. 
