The style of this pamphlet, as well as of others which succeeded it from Jenner’s pen in the course of a few years, is remarkably modest, and admirable in all respects, which probably contributed much to the early favour it received. The facts were such as to defy contradiction, and the conclusions so just and mature, that the experience of nearly forty years has been able to add little more than its seal of confirmation to them. The few errors that have been detected relate chiefly to the degree of protection afforded by the cow-pox, which Jenner affirmed to be perfect: it is now however believed to be incomplete, perhaps in three instances out of every hundred; that small proportionate number passing, in general after the lapse of some years, through a very mild and modified small-pox, in which the per-centage of fatal cases is certainly not more, and probably much less, than five; being not more than three in 2000 of all vaccinated persons, while the rate of mortality even in inoculated small-pox is one in fifty, or forty in 2000. It should be borne in mind that small-pox itself sometimes occurs a second time even in a severe and fatal form, as in the case of Louis XV. Some constitutional peculiarity is probably the occasion of both these anomalies; and this supposition will also account for the often-observed fact, that small-pox after vaccination commonly affects several members of the same family almost simultaneously, thus giving an appearance of failure in a proportion much greater than the truth. Another position advanced by Jenner in this pamphlet is too remarkable to be passed over. After stating his belief that the cow-pox originates from the horse in the way already mentioned, he proceeds to suggest that the small-pox may have been itself originally morbid matter of the same kind, aggravated into a malignant and contagious form by accidental circumstances. But this opinion, though plausible, is not considered by any means as established.