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Aet. XV.— Oti certain remarlmhle Instances of deviation 
from NewtoisPs Scale in the Tints developed by Crystals 
with one Axis of Double Refraction^ on exposure to Polari- 
zed Light. By J. F. W. Heeschel, A. M. F.B. S. Lond. 
& Edin. and of the Came. Phil. Soc. ^ 
JL HE discovery of crystals which possess Two axes of double 
refraction, which we owe to Dr Brewster, is, perhaps, the great- 
est step which has been made in Physical Optics since the disco- 
very of double refraction itself by Bartholin, and its reference 
to an axis by Huygens. It has opened new views on the 
structure of crystals, and will, in all prc^ability, be the means 
of leading us to a more intimate knowledge of the nature and 
laws of those forces, by which the ultimate particles of matter 
act on light and on each other. When we reflect on the situa- 
tion of these axes in different crystallized media, we cannot fail 
to be struck by the variety of the angles they include, and of 
the positions they hold, with respect to the prominent lines or 
axes of symmetry of the primitive molecules, and the question 
immediately suggests itself, What are the circumstances which 
determine their position in the interior of a crystal ? 
It seems to have been all along taken for granted, that what- 
ever these circumstances may be, the nature of the ray must at 
least be a matter of indifference ; in other words, that a red and 
a violet ray similarly polarized, and incident in the same ''direc- 
tion on the same point of a doubly refracting surface, will either 
both undergo, or both not undergo, a separation into two pencils, 
without any distinction arising from the place of the ray in the 
prismatic spectrum. Were this the case, the two axes would be 
fixed lines within the primitive form, absolutely determined by 
the nature of the body, as much so as the lines which bound 
the primitive form itself, and any attempt to substitute for 
them hypothetical axes, coinciding with remarkable lines in 
the latter figure, however ingeniously devised, must be re- 
garded as mere speculation. The fact, however, is other- 
* From the Memoirs of the Cambridge Philosophical Society, vol. i.y which 
will soon be published. This paper was read on the 1st May 1820. 
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