42 SIR DAVID BREWSTER ON A REMARKABLE PROPERTY OF THE DIAMOND. 



a composite crystal consisting of a great number of individual crystals, like certain 

 specimens oi feldspar', carbonate of lime, and other minerals ; but it is more difficult 

 to conceive that the terminal planes of these individual crystals should retain their 

 relative inclination after undergoing the operations of grinding and polishing upon 

 a lapidary's wheel. 



To many persons such a result may appear inadmissible ; but there are several phy- 

 sical facts, which, when well considered, cannot fail to diminish its improbability. 

 If we grind and polish a surface of mother-of-pearl obliquely to the strata of which 

 it is composed, we shall find it impossible to produce a perfectly flat surface : even 

 if we grind it on the finest and softest hone, and polish it with the smoothest powder, 

 the termination of each stratum will remain ; and while the general surface reflects a 

 white image, the grooves or striae will give rise to the beautiful prismatic images 

 produced by interference *. 



Another analogous fact presented itself to me many years ago in examining calca- 

 reous spar. Having had occasion to form an artificial face upon one of the edges of 

 the rhomb containing the obtuse angle, I used a coarse file without water, and found 

 that it exposed faces of cleavage which had never been previously seen, and which 

 were inclined to the general surface produced by the file-f-. 



In examining the optical figures produced by the disintegration of ciystallized sur- 

 faces, I have found that by coarse sandstone, or the action of a rasp, or large-toothed 

 file, we can expose surfaces of crystallization with their natural polish differently in- 

 clined to the general surface J. 



In all these cases the faces, exposed by the mechanical action of grinding or filing, 

 preserve their natural surfaces and polish, and will preserve them more perfectly and 

 readily if they are faces of easy cleavage. The facility of exposing such faces by the 

 action of grinding must increase as the veins or strata become thinner, and it is pro- 

 bable that their exceeding minuteness in the diamond may have aided in the produc- 

 tion of the structure which has been described. 



I have found it quite impossible to measure the inclination of any of the faces by 

 the goniometer ; but I have succeeded, though with some difficulty, in taking an im- 

 pression of the grooved surface upon wax. 



This structure sufficiently explains the existence of three images when the lens was 

 used as a microscope, without supposing that the veins had different refractive 

 powers. Faces of different inclinations would, of course, converge the rays to differ- 

 ent foci on the retina, as effectually as if there had been only a variation in their re- 

 fractive indices. 



* See Philosophical Transactions, 1814. f Edinburgh Journal of Science, Oct. 1828, vol. ix. p. 312. 



X Trans. Royal Soc. Edin. vol. xiv. 



St. Leonard's, St. Andrew's, 

 February Wth, 1841. 



