1889.] 



on Optical Torque. 



489 



you see, a straight line. But now, suppose one of these circular 

 motions to be retarded behind the other, an effect which I can imitate 

 by shifting one of the pins to another position on the disk. Still the 

 resultant motion is a straight line, but it is now executed in a direc- 

 tion oblique to the former. In other words, its plane has been rotated. 

 Of course this model must not be taken as establishing the truth of 

 Fresnel's ingenious theory ; it is at best a rough kinematical represen- 

 tation of it. 



We have, however, the puzzling fact still to account for that there 

 should be two kinds of quartz crystals, right- and left-handed. Sir 

 John Herschel first showed that natural crystals of quartz themselves 

 often indicated their optical nature, by the presence of certain little 

 secondary faces or facets which lay obliquely across the corners of 

 the primary faces. These are indicated in the diagrams (Figs. 12 

 and 13), and may be seen in two of the specimens of quartz crystals 



Quartz crystal, showing characteristic 

 facets : ri^ht-handed. 



Quartz crystal, showing characteristic 

 facets : left-handed. 



which lie upon the table. The largest of these is right-handed. The 

 wider generalisations of Pasteur, respecting the crystalline form of 

 optically active substances, show that those substances which exercise 

 an optical torque, whether as crystals or in solution, belong to the 

 class of forms which the crystallographer distinguishes as i^ossessino- 

 Qon-superposable hemihedry. In other words, they all show sJceio 

 'symmetry, as if in the growth of them they had been built uj) in some 

 screw-fashion around an axis, and must therefore be either right- 

 handed or left-handed screws. By piling up a number of wooden 

 slabs in skew-symmetric fashion, I am able roughly to illustrate 

 ^Figs. 14 and 15) the difference between the right-handed and the 

 left-handed structure. It is a curious fact, if I am rightly informed, 

 :hat down to the present date the only substances possessing this skew 

 symmetry are natural substances ; that those which the chemist can 

 Droduce by artificial synthesis are all optically inactive. It is per- 



I 



