CJiange of Incidence of Small-Pox. 1 27 



Passing to the other limit to which BernoulH thinks 

 that his theory appHes, we find that out of 1,000 living at 

 the end of the first year of life there would be 32 only at 

 the age of 24 who had not already had the small-pox, and 

 even without allowing for deaths by other diseases, only 

 one on an average at the age of 49. On account of the 

 smallness of these numbers Bernoulli does not attempt to 

 alter his theory to deal with them, but limits its application 

 to about 20 years. In the then state of affairs the argu- 

 ment of the small number treated of was a sufficient reason 

 for this neglect ; but for us it is the reason for supposing 

 that the theory could not be pressed further that is most 

 important. I quote it: "Ce n'est qu'au-dessus de cet age 

 qu'on suppose ordinairement la petite verole commencer a 

 devenir un peu plus dangereuse." 



As far as I am able to judge from non-professional 

 literature on the subject, as for instance Mr. Baxendell's 

 paper before this Society, later writers on the subject have 

 been accustomed to speak of small-pox in past times as 

 having been fatal mainly in youth, and not to have been 

 a disease greatly incident on manhood and age. As far as 

 the paper considered is evidence, the facts were different 

 Small-pox appears to have been considered more fatal with 

 advancing years, and the death rate at those ages was only 

 kept low by the immunity conferred by previous attacks of 

 small-pox. 



We must consider to what extent the paper of a 

 mathematician can be considered evidence on this point. 

 It seems to depend on the position and eminence of the 

 writer, who could not afford to be convicted of false repre- 

 sentation in his paper, on the general character of his 

 scientific writings, on the means he was likely to have to 

 obtain the recognised views of the medical profession, and 

 on the fact that, as far as one can gather from the intro- 

 duction written five years later than the paper, the paper 



