166 ON A NEW OPTICAL AND MINERALOGICAL PROPERTY 
mens is represented in Plate IV. Fig. 7. where ABC is the great- 
er angle of the rhomboidal face, and abcd 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 aBFd and JBFc, 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 aBEd, or DBF c, and transmitted Leiuohie ADHE, it is 
divided into two pencils; but these pencils ave suffered such a 
change in passing through the stratum abcd, that when they 
are received upon a spetiall 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 abcd intervenes between a prism abBceFd 
and a flat plate ADHbdcdE; for when the stratum is interposed 
between two prisms, the multiplication of i images takes pi 
as described in the paper already referred to. 
The interrupting stratum a dc d, 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 
