114 Mr. W. Barlow on the Relation of Ciivular Polarization 



For convenience of description let us suppose the axes to 

 be vertical. 



Then wherever one of the specified singular points of the 

 one individual is found next to and vertically over a corre- 

 sponding singular point of the other individual, the parts of 

 one individual will be found turned through 60° as compared 

 with the corresponding parts of the other, and thus they will 

 be related like two succeeding steps of a spiral staircase. 

 And we may suppose that such a disposition, in the case in 

 question, is an effective configuration. 



And since the individual structures thus related are identical, 

 the effective configurations produced in this way will be all 

 identical, and no configuration enantiomorphous to them will 

 be present to set up circular polarization of the opposite 

 hand to neutralize that which they set up. 



As the formation of the effective configurations is an inci- 

 dent of the crystal grouping, they will not be found when the 

 structure is partitioned and dislocated, but only where the 

 twinning competent to produce them exists* 



Class 3. 



The parallel to the case of those of the substances showing 

 circular polarization in the crystalline state only in which the 

 property is inherent, is found in 



Homogeneous structures which contain effective configurations 

 that are not counterbalanced by configurations enantiomorphous 

 to them, and in which, when partitioned and dislocated, these 

 configurations are wanting, their destruction having been brought 

 about by the dislocation. 



An example to illustrate this case can be found in any 

 enantiomorphous type which contains helical structure, the 

 more prominent examples being furnished by those types in 

 which the coincidence-movements due to the presence of 

 helical structure are all of one hand. 



For the helical structures may be supposed to be effective 

 configurations, and when symmetrical partitioning and disloca- 

 tion of the similar fragments take place, they will necessarily 

 disappear *. 



If, with the helical structure, all enantiomorphism dis- 

 appears, so that the fragments of the enantiomorphous 

 structure are, when taken alone, identical with their own 

 mirror-images, it is evident that when a homogeneous struc- 

 ture is reformed from the fragments, it may be either a right- 

 handed or a left-handed enantiomorph. 



* See Min. Mag. 1896, xi. p. 133, 



