transmitted through crystallized Bodies* 1B9 
The pencil of rays to which this remarkable property is 
communicated is surrounded by a large mass of nebulous light, 
which extends about 7° 30' in length, and 1° 7' in breadth on 
each side of the bright image.* This nebulous light never 
vanished with the bright image which it enclosed, but was ob- 
viously affected with its different changes, increasing in mag- 
nitude as the bright image diminished, and diminishing as the 
bright image regained its lustre. From this circumstance I 
was led to conjecture “ that the structure of the agate was in 
“ a state of approach to that particular kind of crystallisation 
“ which affords double images, and that the nebulous light was 
“ an imperfect image arising from that imperfection of struc- 
“ ture.'’ 
On the supposition that this conjecture was well founded, I 
imagined, in conformity with the general analogy of all doubly 
refracting crystals, that the bright image and the nebulous 
light were produced by two different refractive powers, and 
I expected to separate the one from the other by forming the 
agate into a prism with a considerable refracting angle. Every 
attempt of this kind, however, was fruitless: no perceptible 
separation of the images was effected by any of the prisms 
which 1 employed, and I was therefore obliged to abandon this 
mode of investigation. 
Having procured a plate of agate remarkably thin and trans- 
parent, I admitted a beam of light from the sky into a dark 
* On each side of the bright image I have observed a condensation of the nebulous 
light resembling two imperfect images of the luminous body. These imperfect images, 
which increase in number by inclining the agate, are slightly tinged with the prismatic 
colours, which evidently belong to that class of phenomena which have been so abljr 
treated by Dr. Thomas Young, in his late work on Medical Literature. 
