A LABORIOUS YOUTH 27 



nothing but the air intervening between the 

 point of light and our eye, the vibration will 

 not change its direction; but there are certain 

 transparent substances which, when traversed 

 by it, would turn it to the position of the hands 

 of a clock pointing to five minutes to five, if 

 the substance passed through were of a given 

 thickness, and to ten minutes to four if it were 

 double that thickness. In other terms they 

 cause the plane of polarisation to rotate to the 

 left to an extent proportionate to their thick- 

 ness. We will call substances having the power 

 of rotation to the left left substances. There 

 also exist certain right substances, for which, 

 mutatis mutandis, the definition is the same." 

 Young Louis Pasteur entered upon his work 

 in the full midst of the evolution of the sci- 

 ence of crystalography, which led from physics 

 towards chemistry, that was still full of un- 

 solved problems. In pursuing the work re- 

 quired for the last of his university degrees he 

 tried to reconcile those personal studies that 

 were dictated by his individual taste with those 



