W. F. Harvf.y and a. M^Kendrtck 
91 
As a matter of fact the zero index probably has its greatest exactness for very 
thin emulsions, where the number of leucocytes containing no bacteria approximates 
to the modal ordinate in our frequency curve. This leads me to refer again to a 
third measure — the mode — which has been proposed as being more descriptive of 
the phagocytic frequency distribution, and was given by Dr Greenwood (Discussion, 
Pathological Section, Royal Society of Medicine). There is no doubt that it would 
have considerable advantages over the arithmetic mean, but it is a difficult value 
to calculate directly. Further, it would be necessary in every count to set out the 
frequency distribution, and this is not necessary in the estimation of an arithmetic 
mean. We may show these different indices in graphic form. 
A T 1 
Average Index = ■ 
Index 
AB 
A'B' 
Modal Index = ~t7i • 
If, however, we are to search for some one constant more expressive of a 
frequency curve than the mean it seems to me that even more characteristic as a 
measure of dispersion than the mode would be the standard deviation. Our index 
would then consist of the ratio of the standard deviations. We do not know 
exactly why all leucocytes do not take up very nearly the same number of bacteria. 
But the fact is that they do not. The leucocytes, which we may for the moment 
regard as the prime factor, show differentiation as regards phagocytosis, and the 
degree of differentiation seems to depend on the degree of opsonic activity of the test 
sera. We are not to look on this differentiation as something of the nature of 
personal error or error of technique. It would appear to be an integral part of 
the phenomenon of phagocytosis. We should look then to a measure of differen- 
tiation or variability as giving us the best indication of the degree of activity 
which we are endeavouring to estimate quantitatively. For this purpose the 
standard deviation, if the argument is sound, would be a good one to employ. It 
is quite conceivable that within certain limits the standard deviation of the 
