XIV INTRODUCTION. 



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 distilling 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, which has been dwelt upon by 

 M. Eaclot, 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, possessed the power of rotating the plane of 

 polarisation through an angle, dependent on the thick- 

 ness of the plate and the refrangibility of the light. 

 It had, moreover, been proved by Biot that there existed 

 two species of rock-crystal, one of which turned the 

 plane of polarisation to the right, and the other to the 

 left. They were called, respectively, right-handed and 

 left-handed crystals. No external difference of crystal- 

 line form was at first noticed which could furnish a 

 clue to this difference of action. But closer scrutiny 

 revealed upon the crystals minute facets, which, in 

 the one class, were ranged along a right-handed, and, 



