OV THE OPTICAL PECULIARITIES OF THAT MINERAL. 329 



passing through the resultant axes of the sectors A and C, is at 

 right angles to the plane passing through the resultant axes of 

 B and D. In Fig. 18. the tints gradually increase from the 

 centre to the angles A, B, C, D, from the black, or zero of the 

 scale, to the yellow of the first order. In Fig. 19. the tints 

 are uniformly the white of the first order, which is immediate- 

 ly followed, upon inclining the plate, with the most brilliant 

 gamboge yellow, and then green *. 



Having thus described the remarkable structure of Apophyl- 

 lite, I shall now direct the attention of the Society to the ge- 

 neral optical properties of this mineral. 



In the ingenious and elaborate memoir of Mr Herschel, on 

 the action of crystallised bodies on homogeneous light, &c., 

 read to the Royal Society of London on the 23d Decem- 

 ber 1819, he has investigated the origin and nature of the 

 tints which compose the singular system of coloured rings, 

 which I discovered in Apophyllite in 1816. By examining 

 these rings, he found that they " indicated an action on polari- 

 sed light very nearly the same for all the colours, being equal 

 upon the red and indigo blue rays, a little greater for the yel- 

 low and the green, and a little less for the violet ;" and hence 

 he accounted for those unusual tints which characterised this 

 mineral. 



In a subsequent paper, read before the Cambridge Philoso- 

 phical Society on the 1st May 1820 f, Mr Herschel confirm- 

 ed and extended these observations. He found that the law 

 of proportional action was so far subverted in a particular spe- 

 cimen of Apophyllite, that the periods performed by a red ray 



T t 2 were 



* The mechanical structure of the cleavage planes resembles the optical figure 

 even after the planes are ground. 



"t" Published in their Transactions, vol. i. ; and in the Edinburgh Philoso- 

 phical Journal, vol. iv. p. 334. ; and vol. v. p. 334. 



