84 
Dr. Brewster on the 
long diagonal ; and that in general the crystals of carbonate of lime 
which were susceptible of this cleavage, indicated this susceptibility 
by stria* * * § often very strongly marked and parallel to the great dia- 
gonals of two of the opposite rhomboidal faces. * 
The Abbe Haiiy, in his Tableau Comparatif,')' has given the 
name of supernumerary joints to the cleavages observed by Bournon, 
and attempts, without success, to discover their nature and origin, 
In his observations on the simplicity of the laws of crystallisation, J 
he resumes the consideration of them, in reply to the observations 
of Bournon, but he contents himself with stating some variations 
in their appearance and distinctness, which lead him to conclude that 
they are merely accidental. § 
In this state of the subject my attention was directed to the phe- 
nomena of multiplied images which are exhibited in numerous 
specimens of carbonate of lime. These coloured images, which had 
been examined by Dr. Robison, Benjamin Martin, Mr. Brougham, 
and Malus, were ascribed by all these philosophers to cracks or 
fissures in the crystal, by means of which the transmitted light was 
decomposed into the colours of thin plates ; but I have demonstrated 
in two papers on this subject|| that the tints of these images are the 
colours of polarised light produced by thin transparent veins of 
carbonate of lime passing through the long diagonals of the rhom- 
boidal planes. These veins are actual rhomboids of different thick- 
nesses, having their faces placed transversely to those of the 
* Traite de Miueralogie, tom. II. p. 1, 2, 3, 4, 385, &c. Lond. 1808. 
+ Paris, 1809, page 126. 
J Journal des Mines, No. 183. 
§ See Bournon’s Catalogue, p. 489, &c. Lond. 1813, where he combats with much in- 
genuity the opinions of Haiiy. 
|| Phil. Trans, for 1815, p.270, and Edinburgh Transactions, Yol. VIII. p. 165. 
