200 Prof. Reusch on Mica-combinations. 



distinct quadrants by four thin films of mica. In this mica 

 arrangement, however, there occur no superpositions, whereas in 

 the new arrangements it is precisely upon superpositions that 

 the entire matter is founded. Hence there arises conversely the 

 question with respect to that crystalline structure which corre- 

 sponds to a mica- combination with superpositions. 



An idea that naturally suggests itself is as follows — namely, 

 that in the ideal active octahedron of alum the planes of tension 

 parallel to any plane of the octahedron extend only up to the 

 three rectangular axial planes ; but it is also conceivable that 

 the derangement brought about in the original structure of a 

 crystal by means of internal tensions and contractions may be 

 referred to one or to several planes of tension inclined to the di- 

 rection of the transmitted light, and which always traverse the 

 entire crystal in a constant direction. Now we know, it is true, 

 but very little respecting the normal structure of crystals, and 

 consequently even much less with regard to the derangements 

 thereof actually met with; so that if one chooses to take the 

 matter up at all, one finds oneself confined for the present to a 

 few instinctive conjectures. 



In the regular system, in the octahedron of the active alum 

 octahedron, we meet with the case of individual planes of tension 

 that do not pass totally through. The optical phenomena must 

 become more complicated when other planes, those for instance 

 of the leucitoid, occur as distinct planes of tension (leucite, anal- 

 cime ?). The circular polarization of chlorate of soda, discovered 

 by Marbach, is possibly the result of tensions in the direction of 

 the planes of the dodecahedron, combined with secondary tensions 

 in the direction of those of the right or left tetrahedron ; the 

 mica-combination of four systems of plates at an angle of 45°, 

 when the effect of the separate turns is extremely feeble, affords 

 perhaps a representation thereof. 



The same mica-combination corresponds perhaps also to the 

 case of circular polarization in the tetragonal system. Of the 

 four systems of plates, 1 and 3 would to a certain extent re- 

 present the structure of a uniaxial non-rotating crystal; the 

 systems of plates 2 and 4 would be the equivalent of tensions 

 traversing the crystal in totality in the direction of the planes of 

 the right or left hemioctahedron. The occurrence of a single 

 plane of tension inclined to the axis, or the unequal intensity of 

 the individual tensions, would be manifested by the biaxial cha- 

 racter of the optical phenomena (the dislocation of the black 

 cross in beryl, in ferrocyanide of potassium, &c). 



Circular polarization in the rhombohedral system is probably 

 the result of three planes of tension inclined at equal angles to 

 the axis and traversing the entire crystal, these planes agreeing 



