198 



Prof. Reusch on Mica -combinations. 



a combination of mica and selenite, provided only that their co- 



lours do not belong to too high an order. 



Fig. 4. 



In what follows I occupy myself solely with mica plates of equal 

 thickness. Two such plates, wherein, as before, the principal 

 section is to correspond with the longer dimension, can be com- 

 bined to form either a right-ascending stair, A (fig. 3) , or a left- 

 ascending one, B (fig. 4) . Two such stairs possess in any case 

 opposite optical rotation, but the direction of the rotation is deter- 

 mined by the thickness of the plates. If, for instance, the plates 

 give green of the second order, then the right-ascending stair A 

 gives also right-handed rotation ; with plates that give yellow of 

 the first order the contrary is the case. The angle of the prin- 

 cipal sections is without effect with regard to the direction of the 

 rotation ; it must, however, differ to a suitable amount from 0° 

 and 90°. For instance, if two stairs formed of plates crossing 

 at an angle of 60° (A and B) are placed on each other with pa- 

 rallel principal sections, there always remains a rotation in the 

 direction of the uppermost stair. When the stairs are crossed at 

 right angles, no kind of action is produced in the central super- 

 posed portion, and this whatever the azimuth of the combined 

 stair may be. The two arms of the cross arising from the super- 

 position of plates of various stairs are possessed of opposite 

 rotation, 



A case of higher interest, however, is presented when many 

 equal stairs consisting of very thin films of mica are stacked up 

 to form a right- or left-ascending combination. In that case the 

 rotation of the right-ascending combination is right-handed, and 

 that of the left-ascending combination is left-handed. We find 

 therein, therefore, at once the explanation of the triangles «, b, c 

 of the combination crossing at 60°, as represented in fig. 1. The 

 first two belong to a right-handed combination, the last to a left- 

 handed one. The triangles a', b ! , c 1 act, of course, as opposite 

 corresponding ones. By the same rule are determined the di- 

 rections of the rotation in the external points of the star of four 

 systems of plates in a combination crossing at 45°. 



