of Crystalline Reflexion described by Prof. Stokes. 265 



into the cavities, and greatly attenuates the reflecting power. 

 If a corresponding experiment be tried, by wetting the edge 

 of one of Mr. Madan's crystals with saturated solution of the 

 salt, no change is observed to ensue. 



Whether there are cavities or no, the fact that during the 

 preparation the silvery reflexion does not set in until the 

 crystal has sensibly cooled is of great interest. I have found 

 that if a crystal in which the silvery lustre has already been 

 produced be reheated, the lustre disappears, to return again 

 upon a fall of temperature. The operation may be repeated 

 any number of times. 



The existence of twin strata in Iceland spar was observed 

 by Brewster*, and Reusch f has shown that such strata can 

 be induced artificially by suitably applied pressure (fig. 1) in 

 rhombs originally homogeneous. 

 The planes of these strata trun- 

 cate the polar edges, i. e. the 

 edges which meet symmetrically 

 at the obtuse trihedral angle (0). 

 Being desirous of examining 

 whether the reflexion from these 

 strata would conform to the law 

 deduced from theory, I submitted 

 a rhomb to the treatment pre- 

 scribed by Reusch with the effect 

 of developing several exceedingly 

 thin twin laminaB (four or five at 

 least) in close juxtaposition. 



When light is reflected from these strata in a plane perpen- 

 dicular to the edge (0 D) which they truncate, the brilliancy 

 is considerable. But the observation which I wished to make 

 required that the plane of incidence should be perpendicular 

 to this, so as to include the truncated edge and the optic axis. 

 Without much difficulty it was proved that in this plane the 

 reflexion vanished, reviving on either side as the plane of inci- 

 dence deviated a little from the plane of symmetry. The 

 observation was facilitated by immersing the crystal in a 

 small cell containing water or bisulphide of carbon, the twin 

 strata being horizontal, and the plane of symmetry parallel to 

 two of the sides of the cell. 



* Treatise on Optics, 1853, p. 349. 

 t Pogg. Ann. t. xii. p. U6 ^1867). 



