SECOND REPORT —1832. 
340 
capable of being visibly resolved into its elements by the trans¬ 
mission of polarized light. 
And in 1818, ( Edinb. Trans.) Sir David Brewster published 
his representation of the optical structure of analcime, which is in 
some respects even more curious and complex than the prece¬ 
ding cases. The icositetrahedron, which is the usual form,—a 
figure belonging to the class of crystals, which exhibit none of 
the properties connected with polarized light,—is in this spe¬ 
cies distinguishable into 24 solids, of which the boundaries have 
peculiar optical properties. 
This phenomenon of the composition of a crystal, apparently 
simple, of portions exhibiting different optical relations, appears 
in fact to be verv common. Thus nitre and arragonite often 
contain such portions ; and in the second volume of the Cam¬ 
bridge Transactions, Sir David Brewster has shown, that the 
Brazilian topaz possesses a tessellated structure, a central lo¬ 
zenge being surrounded with a border of a different kind, some¬ 
times with additional variations. 
There would be something utterly perplexing in this com¬ 
plexity in the structure of objects apparently so simple, if we 
were to conceive such a kind of composition as formed of inde¬ 
pendent portions adhering together; but we ought, probably, 
rather to conceive these relations of parts as the result of a pe¬ 
culiar state of the equilibrium of the elastic aether which exists 
within the body, and on which its optical properties depend. 
An additional principle, still further complicating the appa¬ 
rently inexhaustible phaenomena of crystals, was discovered and 
fully discussed by Sir J. Herschel {Phil. Trans. 1820). The 
deviation of the succession of colours which many crystals ex¬ 
hibit from that scale of tints which Newton established by obser¬ 
vations on thin plates, and which since his time has always been 
the alphabet of the higher optics, attracted Sir J. Herschel’s 
attention, and he found that it could be fully explained by con¬ 
ceiving the direction of the axis of double refraction to be dif¬ 
ferent for different colours. In biaxal crystals, such a deviation 
is almost universal, as in Rochelle salt, in which it is very pro¬ 
minent. Bicarbonate of potash, indeed, is said to be the only 
biaxal crystal yet examined, in which the axes for all colours 
are found to be strictly coincident. (Herschel, Light, 923.) 
But this deviation from Newton’s scale of colours perplexed 
observers more in the first instance, when it was seen in the 
case of uniaxal crystals. Sir J. Herschel ( Camb. Trans, vol. i. 
Part I.) found certain varieties of apophyllite in which the doubly 
refracting structure was positive for the red, and negative for 
the violet rays, while for the intermediate indigo rays there was 
