The exceeding simplicity of the ultimate discovery makes it difficult for us nowadays to imagine the circumstances under which Jenner had to grope his way in the imperfect twilight, and the perplexities by which he was beset in arriving at true conclusions. Both his own observation and that of other medical men of his acquaintance proved to him that what was commonly called cowpox was not a certain preventive of smallpox. But he ascertained by assiduous inquiry and personal investigation that cows were liable to various kinds of eruptions on their teats, all capable of being communicated to the hands of the milkers; and that such sores when so communicated were all called cowpox. But when he had traced out the nature of these various diseases, and ascertained which of them possessed the protective virtue against smallpox, he was again foiled by learning that in some cases when what he now called the true cowpox broke out among the cattle on a dairy farm, and had been communicated to the milkers, they subsequently had smallpox. It was this repeated failure to arrive at a perfect result which perhaps gave the stimulus that led Jenner to ultimate triumph. The fact that he was on the scent of a discovery which in some form had a promise of indefinite blessing, made him redouble his efforts when most perplexed. He conceived the idea that the virus of the cowpox itself might undergo changes sufficient to deprive it of its protective power, and yet enable it to communicate a disease to the milker. Thus he at last came on the track of the discovery that it was only in a certain condition of the pustule that the virus was capable of imparting its protective power to the human constitution. Having thus steered his way safely through all the pitfalls which might have destroyed the accuracy of his results, Jenner was able to go on to the next stage, that of putting his theory to the test. It is singular how long he was before he had an opportunity of further experiment. In 1788 he showed a drawing of the cowpox as it occurred in milkers to Sir Everard Home and others in London. Various eminent medical men, Cline, Adams, Haygarth, heard of and discussed the matter, and encouraged Jenner’s inquiries. But it was not till May 14, 1796, that he had an opportunity of transferring cowpox from one human being to another. Sarah Nelmes, a dairymaid who had been infected from her master’s cows, afforded the matter, and it was inserted by two surperficial incisions into the arms of James Phipps, a healthy boy about eight years old. The cowpox ran an ordinary course with no ill effect, and in July Jenner writes to Gardner: “The boy has since been inoculated for the smallpox, which, as I ventured to predict, produced no effect. I shall now pursue my experiments with redoubled ardour.”