280 PASTEUR: THE HISTORY OF A MIND 



more analogous than these facts with what was known on 

 the subject of those apparently spontaneous sudden 

 appearances on the horse, on the cow, on the hands of the 

 milkers, that is, those eruptions of horsepox, of cowpox, 

 and of vaccinia? What more natural than to see in 

 smallpox and vaccinia different manifestations of the 

 presence of the same microbe, or at least of two closely 

 related microbes? In all these cases, the ideas regarding 

 microbial diseases and those concerning virus diseases 

 are more and more bound together. Pasteur has just 

 spoken of the imagination. He had much of it, and he 

 allowed it to have full play on this subject. He did 

 not even scorn the dream. "I take the liberty" he 

 said one day, "of recalUng to my confrere, M. Blanchard, 

 that the illusions of an experimenter form a great part 

 of his power. These are the preconceived ideas which 

 serve to guide him. Many of them vanish in the long 

 path which he must travel, but one fine day he discovers 

 and proves that some of them are adequate to the truth. 

 Then he finds himself master of facts and of new prin- 

 ciples, the apphcations of which, sooner or later, bestow 

 their benefits."^ 



The hour had come for him to enter the enchanted 

 grotto full of treasures. 



Ill 

 DISCOVERY OF VACCINES 



The first experiments on chicken cholera date from 

 1879. Interrupted by vacations, they had been resumed, 

 but were upset at once by an unforeseen obstacle. Al- 

 most all the cultures left in the laboratory had become 

 sterile. 



' Comptes rendus de rAcademie des Sciences, !" sem., 1880. 



