88 PROCEEDINGS OF THE AMERICAN ACADEMY 



therefore, is the rhombic prism of 81° 47' in either of the positions 

 we have described, crystallographically compatible with the prism of 

 120°; but also the two diagonals of the first bear to each other pre- 

 cisely the same relation which the lateral axes of a direct hexagonal 

 prism or rhombohedron bear to those of the corresponding inverse 

 forms. 



The prism 81° 47' evidently corresponds to the prism of approxi- 

 mately the same dimensions, on the crystals of antimonious bromide 

 {101}, Fig. 3, Plate I.; and this new condition of antimonious iodide 

 is therefore closely isomorphous with the only known state of antimo- 

 nious bromide and antimonious chloride. 



The rhombic plates of the yellow iodide are quite uniformly marked 

 parallel to the edges and sides of the rhomb, showing an evident ten- 

 dency to the formation of domes and octahedrons, — a phenomenon so 

 familiar in skeleton crystals. 



Although the habit and grouping of the new crystals, the dimen- 

 sions of their angles, and their relations to known forms, furnish very 

 satisfactory evidence of their orthorhonibic character, yet, from the 

 nature of the case, this evidence is not demonstrative, and we were there- 

 fore desirous of obtaining the more conclusive evidence which optical 

 characters give. The crystals, however, obtained as we have described, 

 are usually so excessively thin that we were obliged to search a long 

 time before we could find single plates sufficiently thick to give a dis- 

 tinct interference image. We did, however, at last obtain several 

 plates which enabled us to observe all the important featiu-es of this 

 instructive phenomenon. The hyperbolas were well marked, and sepa- 

 rated by about six divisions of the scale of Groth's polariscope, which 

 corresponds to an apparent angle between the optical axes of about 

 36°, although, on account of the thinness of the plate and consequent 

 indefiniteness of the boundaries of the images, it was impossible to 

 measure the angle exactly. The acute bisectrix was perpendicular to 

 the faces of the plate, at least as closely as could be observed ; the 

 dispersion of the axes was very marked ; and the hyperbolas were bor- 

 dered with green on tlie concave and red on the 

 convex side. Hence, p > ^. No differences could 

 be seen in the coloration between the two ends or 

 the two sides of the image. A very perfect isolated 

 hexagonal plate enabled us to determine that the 

 plane of the optical axes was parallel to the sides 

 of the hexagon, as in most micas, and as repre- 

 •^'S- 4. sented in Fig. 4. This plate was not sufficiently 



