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XXV. On Mica-combinations. 

 By Professor Re use h, of Tubingen*. 



1. TX7HEN one superimposes on each other an even number 

 » » of thin films of biaxial mica in such a manner that 

 the principal sections cross alternately at an angle of 90°, one 

 obtains, even with a moderate number of such crossings, an 

 arrangement the behaviour of which is approximately that of a 

 uniaxial crystal. In the case of the principal sections of the mica 

 coinciding with the crossed planes of polarization, the imitation 

 is perfect ; on turning the arrangement, however, in its plane, 

 the coloured rings remain, but the arms of the black cross be- 

 come bright, and after a rotation of 45° there is left a short- 

 armed cross in the innermost ring only. Norremberg, to whom 

 this experiment is due, was led thereto by the well-known labours 

 of Senarmont upon mica and Rochelle salt. Quenstedt's l Mine- 

 ralogy ' (second edition, p. 239) is, to the best of my knowledge, 

 the only work in which reference is made thereto. It is nowise 

 out of place to remark that Mr. Steeg, optician at Homburg, 

 furnishes these arrangements executed with superior perfection. 

 (See No. 77 of his Catalogue for 1867.) 



2. I have recently occupied myself with certain novel mica- 

 combinations, the object I had in view being the imitation of the 

 behaviour of right-handed or of left-handed uniaxial crystals. I 

 will first of all, with the aid of the figures below, give a descrip- 

 tion of these combinations. 



Fiff. 1. 



Fig. 2. 



Upon two glass plates, pieces of cardboard were cemented 

 wherein three rectangular apertures, crossing each other at an 

 angle of 60°, had been previously cut for the reception of the 

 oblong mica films. The films themselves were derived from a 

 biaxial mica of above 80° between the axes ; they were as thin, 

 and as uniform in their thickness, as possible, and, in all, the prin- 



* Translated from the Monthly Report of the^Berlin Academy of Sciences 

 for Julv 1SGJ), and kindly communicated by W. G. Lettsom, Esq. 



02 



