26 LOUIS PASTEUR 



Some substances have been found to rotate the 

 plane of polarized light to the right and others 

 toward the left. Certain kinds of sugar (dextrose) 

 twist the ray to the right and other kinds such as 

 levulose twist it in the reverse direction, and the 

 amount of these substances in a solution may be 

 ascertained by means of the polariscope by meas- 

 uring the amount of rotation which the light has 

 undergone. 



Is there any relationship between the asymmetry 

 of crystals and the direction in which they rotate 

 the plane of the polarized ray? The English 

 astronomer and physicist, Sir John Herschel, sug- 

 gested that such a relation might exist, but without 

 submitting the question to the test of experiment. 

 It was in regard to this question that Pasteur's 

 researches on the tartrates are of special signifi- 

 cance. It had been observed by Biot that the salts 

 of tartaric acid when in solution rotate the plane 

 of polarized light to the right, but that the so-called 

 paratartrates had no effect on the polarized ray. 

 Pasteur subjected the tartrates to a careful micro- 

 scopic study and succeeded in observing the con- 

 stant occurrence of small facets or surfaces which 

 gave the crystals a slight asymmetry which had 

 escaped previous observers. This asymmetry char- 



