288 On some Properties of Light. 
in the kindred substances of cornelian and chalcedony, and 
it is exhibited in its full effect even when these bodies are 
formed into prisms, and when the incident rays fall with 
any angle of obliquity. In one specimen of agate, which 
has no veins to indicate the direction in which it was cut, 
the images did not vanish as before; and in another speci- 
men of the same character the images suffered only an al- 
ternate diminution of brightness, in the same manner as a 
pencil of light receives only a partial modification when re- 
flected from water at a greater or a less angle than 52° 45’, 
The different experiments which have now been men- 
tioned were repeated, with the most satisfactory results, by 
My. Playfair, Dr. Hope, and Mr. John Davy. 
Although the preceding results are by no means ripe for 
generalization, I cannot omit the present opportunity of 
hazarding a few conjectures respecting the cause of this 
singular property of the agate. 
May not the structure of this mineral be in a state of 
approach to that particular kind of crystallization which 
affords double images? and may not the nebulous light be 
an imperfect image arising from that imperfection of struc- 
ture?) When one of the images vanishes, the nebulous 
light which encircled it is then a maximum, and it gra- 
dually diminishes during the re-appearance of the image. 
When the image which had disappeared recovers its full 
lustre, the surrounding nebulosity is very small; and this 
remaining light is, in all probability, no portion of the un- 
formed image, but merely a few scattered rays arising from 
the imperfect transparency of the mineral. 
By forming the agate into a prism, the nebulous light 
should be separated from the image which it incloses, in 
proportion to the angle contained by the refracting planes 5 
but owing, perhaps, to the smallness of its double refrac- 
tion, if it has such a property, I have not observed any 
separation of this kind. 
The incuryated form of the nebulous light corresponding 
with the curvature of the laminz, seems to connect it with 
the laminated structure of the agate, and to indicate that 
the phenomena of double refraction are produced by an 
alternation of laminz of two separate refractive and di- 
spersive powers. In Iceland spar, one set of the laminz 
may be formed by a combination of oxygen and calcium, 
while the other set is formed by a combination of oxygen 
and carbon. In chromate of lead, the chromium and oxy- 
gen may give one image, while the oxygen and lead give 
another. In like manner the carbonate of lead, the car- 
bonate 
