an artificial Substance rese7nbling Shell, 205 



which exhibits all the variety of colours displayed by thin 

 plates or polarizing laminae. 



The substance is of intermediate hardness between calca- 

 reous spar and sulphate of lime. It scratches the latter easily, 

 and is not scratched by mother of-pearl. Its specific gravity 

 is shown in the following Table, which indicates its relation to 

 analogous substances. 



Calcareous spar 2*72 



Oriental pearls 2*6S 



New substance 2*44? 



Mother-of-pearl ,.,, „. 2-19 



Oyster-shell 2*02. 



The new substance has the property of r^racting light 

 doubly, like most crystallized bodies; and, a^ in agate, mother- 

 of-pearl, &c., one of the two images is perfectly distinct, while 

 the other contains a considerable portion of nebulous light, 

 varying with the thickness of the plate and the inclination of 

 the retracted ray. It has one axis of double refraction, like 

 calcareous spar, which is negative, as in that mineral, and, like 

 it also, it gives a beautiful system of coloured rings by polar- 

 ized light. The double refraction of the substance is very 

 considerable, though greatly less than that of calcareous spar. 

 A plate, one seventy-fifth of an inch thick, makes the first red 

 ring of the system eight inches in diameter at a distance of 

 twenty-six inches from the eye. The substance belongs to 

 the rhombohedral system, and, as in the Chaux carbonatee 

 basee of Hauy, the axis of the rhombohedron, or that of double 

 refraction, is perpendicular to the surface of the thin plates. 

 As mother-of-pearl has two axes of double refraction like ara- 

 gonite, 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 perpendi- 

 cularly to its surface, or along the axis of double refraction, 

 the flame of a candle is seen encircled with a nebulous haze 

 like a halo. By the slightest inclination of the plate in any 

 azimuth whatever, three elongated and curved nebulous images 

 are 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 re- 

 fraction, when all the nebulous light is in one mass, the di- 

 stinct image, D, is redder than in any other direction ; and 

 by slightly inclining the plate the red light disappears, and the 



