SEA-FISHERIES LABORATORY. 135 
the cause of disease outside its own area, and so inland medical 
officers of health will incline to fix on shell-fish as the source 
of infection. Much of the evidence accepted in such cases 
is not convincing. If the cause of illness were made a criminal 
issue it is very doubtful whether a Judge would regard the 
statements made by local sanitary mspectors and medical 
officers of health as legal evidence—at any rate that has still 
to be tested. 
It is quite proper that evidence of marked bacteriological 
pollution should be made the basis of ameliorative work—the 
better disposal of sewage, the erection of purification plant, 
the adoption of systems of cleansing the mussels, voluntary 
certification of “ pure” mussels, sterilisation of the shell-fish, 
etc. But one may reasonably object to analytical results 
being made the basis of penal action, that is, leading to 
restrictions and prohibitions, infraction of which may be 
p) 
punished with fine or imprisonment, unless these results are 
criticised far more severely than has usually been the case 
in the past. ; 
A bacteriological analysis is generally a very definite 
statement—there are so many Bacillus colt per mussel, for 
instance. Scientific men know quite well that any quantitative 
result is always an approximation, and chemists say as much 
when they give the “limits of experimental error.” If the 
data on which a bacteriological result is based are stated, one 
may often find some measure of the degree of error. But 
these data are not always given. There is a single numerical 
estimate of the degree of pollution, and the administrative 
officials for whose use the analysis has been made are left to 
make what allowance they like for error. 
Some methods of estimating the “ margin of error” are 
considered here. These methods are applications of the 
“laws of probability” in relation to samples “taken at 
random.” There is nothing of a very abstract nature about 
K 
