166 ON A NEW OPTICAL AND MINER ALOGICAL PROPERTY 



mens is represented in Plate IV. Fig. 7. where ABC is the great- 

 er angle of the rhomboidal face, and abed the interrupting 

 stratum or film of calcareous-spar. Now, if ABCD were a com- 

 mon specimen of calcareous-spar, a ray of light incident upon 

 either of the faces aQFd and 6BFc, and transmitted through 

 the face ADHE, would be separated into two pencils ; and if 

 these two pencils were received upon another prism of spar, 

 four pencils would be formed, and two of them would vanish 

 at every quarter of a revolution of the second prism. In the 

 preceding specimen, however, when a ray of light is incident 

 upon aBFd, or bBFc, and transmitted through ADHE, it is 

 divided into two pencils; but these pencils have suffered such a 

 change in passing through the stratum abed, that when they 

 are received upon a second prism of common calcareous-spar, 

 none of them will vanish in any position of the second prism, but 

 will continue visible during the whole of its revolution. But if 

 the ray of light is first incident on the surface ADHE, and 

 emerges from aBFd, or bBFc, it possesses the same properties 

 as if it had passed through the common specimens of calca- 

 reous spar. 



The phenomena now described, are exhibited in every spe- 

 cimen of calcareous spar with an interrupting stratum, and cut 

 in the manner shewn in Fig. 7. ; and though they appear at 

 first sight very perplexing, yet they are capable of the most 

 satisfactory explanation. They are visible only when the in- 

 terrupting stratum abed intervenes between a prism abBcFd 

 and a flat plate ADFIbcdE ; for when the stratum is interposed 

 between two prisms, the multiplication of images takes place 

 as described in the paper already referred to. 



The interrupting stratum abed, is crystallised in a different 

 manner from the rest of the rhomboid ; that is, its axes are 

 not placed symmetrically with those of the mass which in- 

 closes 



