284. Sir D . Brewst er on a Remarkable Property of the Diamond. 



viewed by polarized light. Why is it that they exhibit a black 

 cross ? The answer according to the old theory will be, that it is 

 in consequence of the layers varying in density. But what 

 answer will be given to the question, Why do the layers vary in 

 density ? Without calling in aid a new hypothesis, that the 

 layers are formed from within (?), this question cannot be an- 

 swered by the mere formation of layers ; whilst the rolling up 

 and ultimate growing together of the band produces, as already 

 observed, a condensation or pressure of the parts around the so- 

 called kernel. Hence the amylum grain resembles an unannealed 

 glass disc, and consequently in polarized light it must exhibit 

 the coloured cross. 



8. Many additional proofs of my theory might be advanced, 

 but I have no wish to trespass on the patience of the reader, 

 and therefore will leave the discovery of contradictions to my 

 adversaries ; for had I myself found the slightest inconsistency, 

 in a physical point of view, I would at once have rejected the 

 whole theory or admitted its weakness. In conclusion, I will 

 merely observe that, according to my experience, this theory will 

 apply to other kinds of amylum, all of which, as far as I have 

 hitherto seen, produce after boiling the vesicle as described, and 

 in their unfolding perfectly agree with their different kinds of 

 curl. My sincere wish is, that my observations may soon be 

 either fully confirmed or completely refuted ; a proceeding, which 

 cannot, however, be performed by mere arguments, but must be 

 achieved by observation on the stage of the microscope itself. 



XL. On a Remarkable Property of the Diamond. By Sir 



David Brewstee, KM., D.C.L., F.R.S., and V.P.R.S. Ed.* 



[With a Plate.] 



HAVING had occasion, some years agof, to examine the 

 structure of a diamond plano-convex lens which gave 

 triple images of minute microscopic objects, I discovered, by a 

 particular method of observation, that the whole of its plane sur- 

 face 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 dif- 

 ferent reflective, and consequently refractive powers, from which 

 arose all its imperfections as a single microscope. In this case 

 the veins or layers lay parallel, or nearly so, to the axis of the 

 lens, so as to produce the worst effect upon the refracted pencil ; 

 for if the axis of the lens had been perpendicular to the surfaces 



* From the Phil. Trans. 1841, pp. 41, 42. 

 t This Journal, vol. vii. p. 245. 



