26 PASTEUR 



hand, Biot had for a long time been investigat- 

 ing the rotary power of hemihedric crystals, 

 and s he had established that certain of them 

 could deflect polarised light to the right and 

 others to the left. This necessitates an expla- 

 nation which we will borrow from M. Duclaux: 

 "We all know," he writes, "that every lumi- 

 nous impression is the result of a vibration ac- 

 complished after the fashion of a rigid rod 

 which, held in a vise at one of its extremities, 

 vibrates at the other by oscillating around its 

 position of equilibrium. Now, if at the mova- 

 ble extremity it has a polished button reflecting 

 a point of light, we can make this point of light 

 describe an ellipse, a circle or a straight line. 

 Let us examine this last case, which is the sim- 

 plest, and let us agree to give the name of plane 

 of polarisation to the plane which contains the 

 vibrating rod and the line of light described by 

 its extremity. Let us suppose that this plane 

 is vertical and that the point of light is moving 

 before us in line with the hands of a clock 

 pointing to six o'clock. So long as there is 



