38 British Association for the Advancement of Science. 
kneaded 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 structure 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-thirticth 
of an inch, contains many hundred veins oY 
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- 
| the image ; but if these strata are parallel to that axis, 
as they. are in the lens under consideration, each stratum must 
hemes rel med a a oduce a series of wired 
