A SUBSTANCE RESEMBLING SHELL. 
53 
double refraction like aragonite, this new substance may be considered as having the 
same optical relation to calcareous spar that mother-of-pearl has to aragonite. 
When we look through a plate of this substance perpendicularly to its surface, or 
along the axis of double refraction, the flame of a candle is seen encircled with a ne- 
bulous haze like a halo. By the slightest inclination of the plate in any azimuth 
whatever, three elongated and curved nebulous images are Fig. i. 
seen, as in fig. 1 ., the central one, A A, having a distinct 
image, D, of the candle in the middle of it, and the other two, 
B B and C C, being equidistant from A A. These elongated 
images are parallel and concave towards the end of the plate 
nearest the eye. In the direction of the axis of double refrac- 
tion, when all the nebulous light is in one mass, the distinct 
image, D, is redder than in any other direction; and by 
slightly inclining the plate the red light disappears, and the 
distinct image becomes brighter and whiter. All the three 
images, A A, B B, and C C, are united into a mass round D, 
at a perpendicular incidence, but they separate upon inclining 
the plate, and their distance increases with the inclination. 
If we examine the nature of the light of which these images 
are composed, we shall find that the nebulous images, A A, 
B B, are wholly polarized in a plane passing through the di- 
rection of their length, while C C and the greater part of D Bl 
are polarized in an opposite plane. As the thickness of the plate increases, more and 
more of the distinct image, D, is polarized in the same plane, as in mother-of-pearl*, 
till at a certain thickness the whole of it is thus polarized. In this case all the doubly 
refracted light which forms the nebulous image, A A, and the bright one, D, consists 
of two oppositely polarized pencils, the one forming the nebulous and the other the 
distinct image. 
In investigating the cause of these phenomena, we must take as our guide the ana- 
logous facts presented by certain composite crystals of calcareous spar. Having long 
shall only state at present the 
Fig. 2. 
ago described this class of phenomena very fully -f~, I 
general fact. Let A B C D, fig. 2., be a section of 
a rhomb of calcareous spar having its axis perpendi- 
cular to the faces A B, CD, and let E F be another 
crystal, or vein of the same substance crossing it ac- 
cording to the law of crystallographic composition. c 
If we now look at a candle through this compound crystal, it will appear single in 
the direction of the axis of A B C D ; but if we incline the plate Fig. 3. 
in a plane passing through A B, we shall see two images to- B / , A |j| D |j|c 
gether as at A and D, fig. 3., and other two, namely one at B 
* See Philosophical Transactions, 1814. 
f Id., 1815. Edinburgh Encyclopedia, art. Optics. 
