14 SEVENTH REPORT—1837. 
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 delicate microscopical observations, produced no duplica- 
tion 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 images 
might arise from some other cause. He therefore proceeded to ex- 
amine the light transmitted through the diamond, by combining 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 capable 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 reflected the light more powerfully than others, so as to 
have the appearance of a striped riband, 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, 
contaius many hundred veins or strata of dif- 
ferent reflective and refractive powers, as if they 
had been subjected to variable pressures, or de- 
posited under the influence of forces of aggrega- 
tion of variable intensity. If, Sir David observed, 
the planes of these different strata had been per- 
pendicular to the axis of the diamond lens, their 
difference of refractive power would produce no 
sensible effect injurious to the perfection of the 
image; but if these strata are parallel to that axis, as they are in the 
lens under consideration, each stratum must have a different focus, and 
consequently produce a series of partially overlapping images. 
The results of this experiment in restoring the diamond to its value 
as an optical material, in so far as it enables us to cut it in a proper 
direction, and select proper specimens, and its connexion with some 
delicate researches of Professors Airy and Maccullagh on the super- 
ficial action of diamond upon polarized light, possess considerable in- 
terest; but the fact of a mineral body consisting of layers of different 
refractive powers, and consequently different degrees of hardness and 
specific gravity, is remarkable. There were several minerals, such as 
Apophyllite, Chabasie, and others, in which Sir David had found differ- 
ent degrees of extraordinary refraction in different parts of the crystal ; 
but this variation of property depends upon a secondary law of struc- 
ture; and he believed that there was no crystal, either natural or 
artificial, in which the properties of ordinary refraction, hardness, and 
specific gravity, varied throughout its mass. This peculiarity of strue- 
ture, therefore, might be regarded as an indication of a peculiarity of 
