8 PASTEUR: THE HISTORY OF A MIND 



we are dealing the progress came by way of physics 

 from the introduction into the questions of mineralogy 

 of the power to rotate the plane of polarization. 



We know that every impression of light is the result 

 of a vibration. It is as though a rigid rod, clamped in a 

 vise at one end, should vibrate at the other end, oscill- 

 ating about a position of equilibrium. If, on the moving 

 end, there is a polished button making a luminous point, 

 we can describe with this luminous point an ellipse, 

 a circle, or a straight line. Let us consider this last 

 case, the simplest one, and let us call, for sake of argument, 

 the plane of polarization the plane which contains 

 the vibrating rod and the luminous line which its 

 extremity describes. Let us suppose this plane verti- 

 cal, and the luminous point moving before us in the 

 line occupied by the hands of a clock indicating six 

 o'clock, i.e., in a vertical line. As long as only the 

 air intervenes between the luminous point and our 

 eye the vibration will not change direction, but there are 

 many transparent substances which, when traversed 

 by the vibration, would make it project itself along 

 the lines of the hands of a clock indicating five minutes 

 of five for a certain thickness traversed, or ten minutes 

 of four for a thickness twice as great. In other words, 

 these substances rotate the plane of polarization to the 

 left an amount proportional to their thickness. We 

 call them substances having a left rotary power, or, to 

 abbreviate, left-handed substances. There exist, further- 

 more, right-handed substances, of which, mutatis mutandis, 

 the definition is the same. 



Crystallized quartz, the hemihedral form of which 

 we have just seen, is typically one of these substances 

 endowed with rotary power; it rotates the plane of 

 polarization of a ray of light which traverses it in the 

 direction of the axis, and Biot, in the very careful study 



