Sir D. Brewster on a Remarkable Property of the Diamond. 285 



of these veins, its performance as a microscope would scarcely 

 have been injured by them. 



In repeating Mr. Airy's experiments on the action of the dia- 

 mond in modifying Newton's rings near the polarizing angle, I 

 was led to re-examine the flat surface of the diamond above 

 mentioned; but though I found my former observations per- 

 fectly correct, yet I was induced to suspect the accuracy of the 

 inference which I drew from them, and which I could not but 

 draw in the circumstances under which the phEenomenon was 

 presented to me. 



In order that the Society may be able to judge of the new 

 results at which I have arrived, I have given in Plate VI. fig. 4 

 as accurate a drawing as I am able to make of the appearance of 

 the flat surface of the diamond under consideration, as seen by 

 light incident upon it nearly perpendicularly. The flat surface 

 of the diamond is # 058, or y^th of an inch in diameter, and 

 owing to the great convexity of its other surface, the light re- 

 flected by it does not interfere with the examination of the struc- 

 ture above mentioned. 



The appearance shown in the figure is that which I observed 

 some years ago ; but upon shifting the line of illumination, I 

 was surprised to perceive that all the dark bands became light 

 ones, and all the light bands became dark ones, a phenomenon 

 which placed it beyond a doubt that all the bands were the edges 

 of veins or lamina whose visible terminations were inclined at dif- 

 ferent angles, not exceeding two or three seconds to the general sur- 

 face. Had this surface been an original face of the crystal there 

 would have been nothing surprising in its structure, excepting 

 the exceeding minuteness of the strata and the slight inclination 

 of their terminal planes to each other ; but being a surface 

 ground and polished by art, the phenomenon which it presents 

 is one extremely interesting. 



The mineralogist will have no hesitation in admitting that this 

 diamond is part of a composite crystal consisting of a great 

 number of individual crystals, like certain specimens of felspar, 

 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 ope- 

 rations of grinding and polishing upon a lapidary's wheel. 



To many persons such a result may appear inadmissible ; but 

 there are several physical 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 fiat 

 surface : even it' 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 



