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2. The detective method used. In the ordinary methods the 
blood is spread out in a thin film, liquid or dry, one cubic millimetre 
covering say four square centimetres of area. If the field of the 
oil-immersion lens has a diameter of 0165 mm. and can be 
thoroughly searched by the eye in three seconds, about twelve hours' 
work would be required to examine the whole of 1 c.mm. of blood 
in this way; and if the specimen contains only one parasite it may 
not, by bad luck, be found until nearly the whole of the area has 
been searched. We have therefore employed the ‘ thick film process' 
described by one of us (R.R.) in 1903. In this 1 c.mm. of blood is 
spread out over only about one-quarter of a square centimetre or 
less, and dried ; the haemoglobin is then gently washed out with 
water; and the residue, consisting of parasites, leucocytes, platelets, 
and the stromata of the red corpuscles, is carefully stained by any 
convenient method, with or without fixation. By this simple means 
the whole of a cubic millimetre of blood can be searched in less than 
an hour ; but the identification of the Plasmodia requires considerable 
practice. 
3 - 7 he enumerative method used. The younger Plasmodia are 
too small to be counted by the use of an ordinary haemocytometer; 
hence their number has usually been estimated by comparison with 
the number of red or white cells, the latter figure being determined 
by the haemocytometer. This gives a large compound error; for 
if e is the percentage error made in counting the red or white cells 
in one specimen, and e 1 is the percentage error made in computing the 
proportion of parasites to each corpuscle in that specimen, then, by a 
simple calculation, the total percentage error will be e + e 1 + ee 1 / 100- 
(For example, if e= +5% and e'= + 10%, the total percentage error 
will be + 15-5%.) 
Our method consists simply in making a measured quantity of 
00 into a thick film preparation and then counting all the 
parasites in it. The blood is measured by determining the diameter 
a fine capillary tube under the microscope, and then calculating 
and marking off the length of the tube required to hold a given 
quantity (as suggested by Dr. Wakelin Barratt). Thus a uniform 
ube of 0180 mm. diameter and 4 cm. length will contain 
c.mm. a convenient size. The blood is sucked into the tube 
