484 
Proceedings of Royal Society of Edinburgh. [sess. 
Statistical Studies in Immunity : The Theory of an 
Epidemic. By John Brownlee, M.D. (Glas.). Com- 
municated by Dr R. M. Buchanan. 
(Read June 18, 1906.) 
The rise and decline of epidemics of infectious diseases have 
been subjects of interest since the earliest times, but the scientific 
determination of the laws which govern their course offers even 
yet a wide and almost unworked field. Not but what a large 
amount of observation has been made on many of the conditions 
under which epidemics appear and pass away. Many epidemics 
are seasonal, and these have been studied; but the lack of any means 
of determining the course which a given epidemic might have 
taken in the presence of somewhat different conditions has made 
the deduction of certain conclusions impossible. Even the laws 
which regulate solitary outbursts of disease, the special subject of 
this paper, have been little studied. Explanations offered have 
varied with the period in history. We find that the pestilence 
which afflicted Israel for David’s sin stayed at the threshing-floor 
of Araunah the Jebusite ; we find in the Iliad that with the 
return of Chryseis “ the dread clang of Apollo’s silver bow ” 
ceased ; later, the great fire of London is commonly believed to 
have exterminated the plague, while at this moment the hasty 
cleansing of a town by a terrified sanitary authority is by many 
thought to be the direct cause of the disappearance of an epidemic ; 
but in all these cases there is a misinterpretation of facts, which is due 
largely to the absence of any real knowledge of the underlying laws. 
The most important contribution to the subject from the point 
of view of this paper is one by the late Dr Farr, who had that 
genius which permitted him to perceive a large part of the laws 
which govern progressions of figures. In 1866, when the cattle 
plague was making most extensive ravages in this country, and 
when, from the rate of its progress, there seemed no end to the 
damage it might do, he wrote a letter in which he showed that, as 
the rate at which the disease was extending was already lessening, 
the acme and the decline of the epidemic might soon be expected. 
