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On the Optical Figu7'es produced hy the Disintegrated Surfaces of Crystals. 

 By Sir David Brewster, K.H., D.C.L., F.R.S. 



Read 6th February 1837. 



There is no branch of natural science about which we know so little as that 

 which relates to the structure of crystalline bodies. By assuming the form of an 

 integrant molecule, crystallographers have found no difficulty in building those 

 geometrical solids which minerals and artificial crystals present to our observation. 

 They conceive that these molecules unite by their homologous sides in the foiTaa- 

 tion of the primitive crystal, and by supposing that they arrange themselves in 

 plates on the faces of that crystal, each plate successively diminishing in size 

 by the abstraction of a certain number of these molecules in lines of a given direc- 

 tion, — all the secondary forms of the crystal may be easily deduced. 



In place of employing, as Hauy has done, integrant molecules having the 

 form of a tetrahedron, a triangular prism, and a parallelopiped, others have sug- 

 gested the more philosophical idea of constructing crystals out of spheroidal ele- 

 ments, including, of course, the sphere by which the oblate passes into the prolate 

 solid. But in whatever way crystallographers shall succeed in accounting for the 

 various secondary forms of crystals, they are then only on the threshold of their 

 subject. The real constitution of crystals would be still unknown ; and though 

 the examination of these bodies has been pretty diligently pursued, we can at 

 this moment form no adequate idea of the complex and beautiful organisation of 

 these apparently simple structures. The double refraction and p3rro-electricity of 

 crystals related to certain fixed points of theh primitive forms ; and the pheno- 

 mena of circular polarisation in quartz and amethyst, connected with the plagiedral 

 faces of the crystal, indicate remarkable peculiarities of structiu'e ; and I have 

 had occasion to shew that all the properties comprehended under Double Refrac- 

 tion and Polarisation do not exist in the ultimate molecules of the body, but are 

 wholly the result of those forces by which these molecules are combined. Struc- 

 tures still more complicated have been discovered by the analysis of polarised 

 light, and in the complex formations of apophyllite and analcime, we witness the 

 operation of laws resembUng more those which regulate the structures of animal 

 life, than those which had previously been observed in crystalline formations. 



The doubly refracting structure of crystals, or to use the language of the un- 

 dulatory theory, the law according to which this structure permits the ether to 



