176 LOUIS PASTEUR, 



teur, speaks with such emphasis about 'preconceived 

 ideas,' he cannot mean ideas without antecedents. 

 Preconceived ideas if out of deference to M. Pasteur 

 the term be admitted are the vintage of garnered 

 facts. We in England should rather call them induc- 

 tions which, as M. Pasteur truly says, inspire the mind 

 and shape its course in the subsequent work of deduc- 

 tion and verification. 



At the time when M. Pasteur undertook his inves- 

 tigation of the diseases of silkworms, which led to 

 such admirable results, he had never seen a silkworm ; 

 but so far from this being considered a disqualifica- 

 tion, his friend M. Dumas regarded his freedom as a 

 positive advantage. His first care was to make him- 

 self acquainted with what others had done. To their 

 observations he added his own, and then, surveying 

 all, came to the conclusion that the origin of the 

 disease was to be sought, not in the worms, not in the 

 eggs, but in the moths which laid the eggs. I am not 

 sure that this conclusion is happily described as *a 

 preconceived idea.' Every whipster may have his 

 preconceived ideas ; but the divine power, so largely 

 shared by M. Pasteur, of drilling from facts their 

 essences of extracting from them the principles from 

 which they flow is given only to a few. 



With regard to the discovery of crystalline facets in 

 the tartrates, dwelt upon by M. Radot, a brief reference 

 to antecedent labours may be, here allowed. It had been 

 discovered by Arago, in 1811, and by Biot, in 1812 and 

 1818, that a plate of rock-crystal, cut perpendicular 

 to the axis of the prism, and crossing a beam of plane 

 polarised light, caused the plane of polarisation to 

 rotate through an angle, dependent on the thickness of 

 the plate and the refrangibility of the light. It had, 



