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IV. On a Remarkable Property of the Diamond. 

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



Received February 15, — Read March 4, 1841. 



Having had occasion, some years ago, to examine the structure of a diamond 

 plano-convex lens which gave triple images of minute microscopic objects, I disco- 

 vered, by a particular method of observation, that the whole of its plane surface was 

 covered with hundreds of minute bands, some reflecting more and some less light ; 

 and I naturally drew the inference that this diamond consisted of a great number of 

 layers of different reflective, and consequently refractive, powers, from which arose 

 all its imperfections as a single microscope. In this case the veins or layers lay par- 

 allel, or nearly so, to the axis of the lens, so as to produce the worst eff*ect upon the 

 refracted pencil ; for if the axis of the lens had been perpendicular to the surfaces 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 diamond in modifying 

 Newton's rings near the polarising angle, I was led to re-examine the flat surface of 

 the diamond above mentioned ; but though I found my former observations perfectly 

 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 pheno- 

 menon 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 I. fig. 1. 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 0*058, or 

 "i^T-th of an inch in diameter, and owing to the great convexity of its other surface, 

 the light reflected by it does not interfere with the examination of the structure 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 laminw whose 

 visible terminations were inclined at different angles, not exceeding two or three seconds 

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

 would have been nothing surprising in its structure, excepting the exceeding minute- 

 ness 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 



MDCCCXLI. ^ G 



