Prof. Keusch on Mica-combinations. 199 



These stacked-up combinations, or piles, exhibit also another 

 peculiarity : in converging light there is seen through the su- 

 perposed portion a biaxial system, the principal section of which 

 bisects the acute angle of the principal sections of the mica films, 

 and the angle subtended by the axis of which is less than that of 

 the mica employed. The black hyperbolas, however, appear only 

 when the principal section of the mica-combination forms an 

 angle of 45° with the planes of polarization ; in the case of its 

 coinciding with either the one or the other, the innermost rings 

 exhibit merely black spots. 



The effect of a pile thus built up may be imitated with some- 

 thing like approximation by means of a thick plate of mica so 

 inserted between two eighth-of-undulation plates, whose axes 

 are crossed at a right angle, that the principal section of the 

 mica plate makes an angle of 45° with them ; and this combina- 

 tion has a different direction of rotation in parallel light accord- 

 ing as the principal section of the plate bisects the one or the 

 other pair of the vertical quadrants formed by the principal sec- 

 tions of the eighth-of-undulation plates. This reminds one of 

 an arrangement described by Dr. John Miiller, in the seventh 

 edition of his ' Natural Philosophy ' (vol. i. p. 906), wherein, by 

 means of an analogous combination of a selenite plate with two 

 quarter-undulation mica places, the effect of quartz is imitated, 

 at least in parallel light. 



4. The optical effects of the mica- combinations above described 

 admit of mathematical treatment ; and it is to be presumed the 

 phenomena in parallel light will not present anything like the 

 difficulty which those will offer when converging light is used. 

 Perhaps some competent calculator may be induced to occupy 

 himself with this not altogether thankless problem. 



The future will teach us whether these combinations are cal- 

 culated to afford us an explanation, or at least hints, respecting 

 the connexion of particles endowed with circular polarization. 

 At present in this respect I can only adduce a feeble analogy 

 and a few suggestions, to which 1 would direct the attention of 

 those who are learned in such matters. 



In a former communication upon the so-called lamellar pola- 

 rization of alum (see General Meeting of the Academy, July 11, 

 1867), I have endeavoured to establish that what we have to do 

 with here is a feeble double refraction due to internal tensions, 

 which must be conceived to be so acting in the planes of the 

 octahedron that the optical elasticity is equal in those planes in 

 all directions, but less than at right angles thereto. I haev 

 further shown how the effect of an optically active octahedron of 

 alum, or of a preparation therefrom in the direction of two pa- 

 rallel planes of the cube, may be perfectly imitated in the four 



