Dr. Brewster on the multiplication of images, &c. 271 
In this state of the subject, my attention was accidentally 
directed to it, in consequence of having received, from Sir 
George Mackenzie, Bart., some very fine specimens of cal- 
careous spar, which possessed the property of multiplying 
and colouring the images. I examined with attention the 
different planes in which the images were polarised ; I found 
that small crystals detached from particular parts of the spe- 
cimens, possessed the same properties, and I represented in 
figures the specimens which I employed, and the interrupt- 
ing planes by which the colours were obviously produced. 
These results convinced me, that the interrupting plane was 
not a fissure or fracture ; and I conjectured that the colours 
were analogous to those produced by the action of crystals 
upon polarised light.* By following out this conjecture, I have 
been led to the true cause of all the phenomena, and of other 
analogous facts ; and have thus been enabled to communicate 
to any specimen of Iceland spar, the faculty of multiplying 
and colouring the images, in a manner so exactly similar to 
the real specimens, that no person can discern the least diffe- 
rence between the phenomena of the artificial, and those of 
the natural crystal. The results to which this explanation 
leads, will, I trust, be equally interesting to the mineralogist 
and to the natural philosopher. 
Sect. I. On the phenomena exhibited by particular specimens of 
Iceland spar. 
Let AEBFHDGC, (PL XV.) fig. 1. be a rhomboid of calca- 
reous spar, and let the supposed fissure by which the coloured 
images are produced, be in the plane ABCD. When a pencil of 
* Treatise on new Philosophical Instruments, Sec. p. 339, and Pref. p. xii. 
