16 Sir David Brewster on the Optical Figures produced by 



seen at Bursin sur Nolle, a distance of 16,000 metres. In public 

 illumination, however, on account of the turnings of streets and 

 lanes, it is necessary to multiply the illuminating points ; and it 

 has ever been an objection to the electric light, that its iHunG&Ha- 

 tion can only be local. The Professor has, however, assured 

 himself that more than one pair of coal-points may be introduced 

 into the self-same circuit, and a light intolerable to the eye 

 obtained at each place of interruption. Possibly by an extension 

 of this principle, and the introduction of suitable commutators 

 for opening and closing the circuit, the ligh't ra% be made 

 available for domestic purposes. Not only has the light been 

 found sufficient to produce a portrait on a Daguerreotype plate, 

 but Prof. Wartmann has proved that even the image of the green 

 portions may be obtained. 



VI. On the Optical Figures produced by the Disintegrated Sur- 

 faces of Crystals. By Sir David Brewster, K.H., D.CJAj^ 

 F.R.S.* 



[With Five Plates.] >lMOh Hih 



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 mole- 

 cule, 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 formation 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 direction, — 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 suggested the more philosophical 

 idea of constructing crystals out of spheroidal elements, 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 secondaiy forms of crystals, they are 

 then only on the threshold of their subject. The real constitu- 

 tion of crystals would be still unknown ; and though the exami- 

 nation of these bodies has been pretty diligently pursued, we can 

 at this moment form no adequate idea of the complex and beau- 

 tiful organization of these apparently simple structures. The 

 double refraction and pyro-electricity of crystals related to cer- 



* From the Transactions of the Royal Society of Edinburgh, vol. xiv. 

 p. 1 ; having been read February 6, 1837. 



