322 Races of Entamoeba histolytica 
therefore to those cysts which are perfectly spherical, and thus capable 
of exact and rapid measurement, we should ignore the vast majority of 
cysts in any infection; a procedure which might lead to a totally false 
estimate of the dimensions of the cyst population as a whole. Accord¬ 
ingly we have ignored only those cysts which were much misshapen and 
which formed only a very small proportion of the total number in the 
infections here described; and we have measured all those which were 
approximately elliptical or circular in outline. The size of the latter 
was recorded simply as the diameter, and of the former as the mean of 
the longer and shorter axes. 
All the measurements were made with a finely-ruled ocular micro¬ 
meter, in combination with a i/12" oil immersion objective. The micro¬ 
scope was adjusted in all cases so that one division of the micrometer 
scale represented T5p.. We always estimated the diameter of a cyst to 
the nearest half of a division, so that the unit employed was in reality 
0-75p,. All the measurements were recorded directly in units of the 
micrometer scale, and not converted into microns until afterwards. 
Perfectly accurate measurements cannot of course be obtained by this 
method, but we know of none better for obtaining a large series of fairly 
trustworthy measurements within a reasonable time. 
Since every measurement made in this manner involves a certain 
amount of estimation on the part of the measurer, it is clear that the 
results recorded by diherent individuals are likely to differ slightly from 
one another. We endeavoured to determine our own individual differ¬ 
ences in this respect in the following way. One hundred cysts in a fixed 
and stained preparation w^ere measured as carefully as possible, each 
cyst being measured by each of us independently, and the measurements 
independently recorded. On comparing the results we found that the 
dimensions registered were identical for 71 of the cysts. For each of the 
remaining 29 cysts there was a difference of half a micrometer division. 
