1905 - 6 .] Studies in Immunity : Theory of an Epidemic. 519 
The symbols used in the paper and table are as follows : — 
/* 2 ’ /* 3 » t 1 4 represent the second, third, and fourth moments of 
the curve round its centre of gravity, and are obtained by multi- 
plying each vertical strip by its corresponding abscissa, squared, 
cubed, or raised to the fourth power respectively, and dividing by 
the area of the curve. 
For the determination of which curve is to he used in fitting 
the sign of the quantity 6 + 3/3 1 - 2/3 2 is important, where 
2 
(3 1 = — o and /3o = — • ■ For curves of type IV. the sign of this 
quantity is negative. 
The other constant of interest, as indicating the asymmetry of 
the curve, is d, which is the abscissal distance of the maximal 
ordinate from that through the centre of gravity. 
The value of the other constants will he found in Professor 
Pearson’s papers in the Philosophical Transactions 
[Tables. 
