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giving to Chappuis a vague notion of some piece of crystal 

 in a glass case, but was absolutely evoking a vision of the beauti- 

 ful crystal, perfectly pure and transparent, brought from Ice- 

 land in 1669 to a Danish physicist. Pasteur almost seemed to 

 experience the surprise and emotion of this scientist, when, 

 observing a ray of light through this crystal, he saw it sud- 

 denly duplicated. Pasteur also spoke enthusiastically of an 

 officer of -Engineers under the First Empire, Etienne Louis 

 Mains. Malus was studying double refraction, and holding in 

 his hands a piece of spar crystal, when, from his room in the 

 Rue de 1'Enfer, it occurred to him to observe through the 

 crystal the windows of the Luxembourg Palace, then lighted 

 up by the setting sun. It was sufficient to make the crystal 

 rotate slowly round the visual ray (as on an axis) to perceive 

 the periodic variations in the intensity of the light reflected 

 by the windows. No one had yet suspected that light, after 

 being reflected under certain conditions, would acquire proper- 

 ties quite different from those it had before its reflection. 

 Malus gave the name of polarized light to light thus modified 

 (by reflection in this particular case). Scientists admitted in 

 those days, in the theory of emission, the existence of luminous 

 molecules, and they imagined that these molecules " suffered 

 the same effects simultaneously when they had been reflected 

 on glass at a certain angle. . . . They were all turned in the 

 same direction." Pouillet, speaking of this discovery of Malus 

 in the class on physics that Pasteur attended, explained that 

 the consequent persuasion was ' ' that those molecules had rota- 

 tory axes and poles, around which their movements could be 

 accomplished under certain influences." 



Pasteur spoke feverishly of his regrets that Malus should 

 have died at thirty-seven in the midst of his researches ; of 

 Biot, and of Arago, who became illustrious in the path opened 

 by Malus. He explained to Chappuis that, by means of a 

 polarizing apparatus, it could be seen that certain quartz 

 crystals deflected to the right the plane of polarized light, whilst 

 others caused it to turn to the left. Chappuis also learned that 

 some natural organic material, such as solutions of sugar or of 

 tartaric acid, when placed in such an apparatus, turned to the 

 right the plane of polarization, whilst others, like essence of 

 turpentine or quinine, deflected it to the left ; whence the ex- 

 pression " rotatory polarization." 



These would seem dry researches, belonging altogether to 



