38 British Association for the Advancetnent of Science. 



kiieaded together like a piece of soft gum or an indurated jelly, he 

 had no doubt that the double images were owing to this structure, 

 as there appeared, on an ordinary examination of the lens, to be 

 no other cause to which it could be reasonably ascribed. This 

 was also Mr. Pritchard's opinion, and the existence of such images 

 prevented opticians from rashly cutting up diamonds which might 

 turn out useless for optical purposes. As lenses of sapphire and 

 ruby, which Sir David had long had occasion to use in very deli- 

 cate microscopical observations, produced no duplication of the 

 image, although the rays passed in directions in which the double 

 refraction was much greater than in any specimen of diamond 

 which he had examined, it occurred to him that the double im- 

 ages might arise from some other cause. He therefore proceeded 

 to examine the light transmitted through the diamond, by com- 

 bining it with a concave lens of the same focal length, in order 

 to make the rays pass in parallel directions through its substance. 

 This experiment indicated no peculiarity of structm^e at all capa- 

 ble of producing a separation of the images, and he was therefore 

 led to examine the plane surface of the lens by reflecting from it 

 a narrow line of light admitted into a dark room, and examining 

 the surface with a half-inch lens. While turning round the plane 

 surface of the diamond, he was surprised to observe the whole of 

 its surface covered with parallel lines or veins, some of which re- 

 flected the light more powerfully than others, so as to have the 

 appearance of a striped ribband, somewhat 

 resembling the rude sketch here given, 

 which shows that the plane surface of the 

 diamond, in a space of less than one-thirtieth 

 of an inch, contains many hundred veins or 

 strata of different reflective and refractive 

 powers, as if they had been subjected to va- 

 riable pressures, or deposited under the influ- 

 ence of forces of aggregation of variable intensity. If, Sir David 

 observed, the planes of these different strata had been perpen- 

 dicular to the axis of the diamond lens, their difference of refrac- 

 tive power would produce no sensible effect injurious to the per- 

 fection £>f the image ; but if these strata are parallel to that axis, 

 as they are in the lens under consideration, each stratum must 

 have a diff"erent focus, and consequently produce a series of par- 

 tially overlapping images. 



