44 Prof. Maskclyne and Dr. Lang's Miner alogical Notes* 



is zero for mesotype : placed in a cone of polarized light, we also 

 rind that in mesotype the first mean line must be parallel to the 

 prism. Mesolite, on the contrary, shows in polarized light cer- 

 tain phenomena of interference, which were first observed by 

 Descloizeaux, and which are certainly due to the complicated 

 twin structure of these crystals. On submitting a prismatic 

 crystal from a specimen which is labelled Brevicite in the minera- 

 logical collection of the British Museum, to the test of polarized 

 light, I found that the plane of polarization was indeed parallel 

 to the prism, but that the plane of the optic axes went across 

 that direction ; and it was possible even to see one of the optic 

 axes next the limit of the field of view. 



The prism of this crystal, therefore, cannot be identical with 

 the prism hitherto known on mesotype, but must be parallel to 

 the axis a. I measured several similar crystals, and found that 

 their planes are in simple relation to the planes which are already 

 known on mesotype. What convinced me still more that I had 

 to deal with crystals of this species was, that I found on the 

 back of the same specimen other crystals, which, although exhi- 

 biting the well-known character of combination, showed also 

 several of the new planes previously found on the former kind 

 of crystals. 



The prism parallel to the axis a would be expressed by the 

 symbol (0 31) with the parameters used in the mineralogical 

 treatise of Brooke and Miller. But as this form determines the 

 character of combination, and as I even found a crystal twinned 

 on this plane, I think it more convenient to give to it the simpler 

 symbol (Oil), multiplying the parameter c with 3, which does 

 not alter the symbol of the face (110). 



For mesotype, therefore, the symbols of the known faces 

 become 



a (100) m(110) o(113) ?/(313), 



and those of the new faces 



£(010) c(001) w(210) A(101) u{01l) *(111)/(311). 



Fig. 5, Plate III. shows the principal zones formed by these 

 faces. Using the following angles given by Brooke and Miller, 



ma = 45 30, 

 mo = 63 20, 



we find for the crystallographic elements the values 



«:&:c=0'9470: 0-9306:1. 



With these elements are calculated the angles of the following 

 Table:— 



