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being positive, like zircon, and forty negative, like calcareous spar. 

 The phenomena observed are most splendid, and open up a wide field 

 of research. 



The author next detailed, and illustrated with minute and carefully- 

 coloured drawings, these phenomena, as observed in the following 

 substances, the most remarkable of the whole number. 



1. Lithoxanthate of Ammonia. — Here, in the usual specimens, 

 the light polarized by the sectors is the blue of the first order, 

 often the white and the yellow of the same order. In separate cir- 

 cular crystals, other appearances occur. In one, the three first 

 orders of colours appeared exactly as in the coloured rings of uni- 

 axial crystals, proving that the elementary prisms or radii must have 

 increased in thickness from the centre outwards, according to New- 

 ton's law of periodical colours. In others, the second and third 

 bands were of different but uniform colours throughout, proving 

 uniform thickness all round in each band. These colours were 

 generally red and green, not at all related to the central tint, or 

 to one another. In some cases, the order of colours is inverted. 

 In the most perfect crystals, the central tints are the blue and white 

 of the first order, in consequence of the great minuteness of the ele- 

 mentary crystals, which form a more uniform disc, with an exceedingly 

 sharp black cross. This central part is surrounded by a narrow black 

 ring, beyond which is an annulus of sectors, sometimes white, like 

 the inner ones. This is terminated by a black circle, beyond which 

 is a third series of sectors, either white or blue of the first order. The 

 black cross starts into greater breadth as it passes from one annulus 

 to the other, from the inferior degree of optical contact in the outer 

 rings. Various other singular modifications occur in this salt, which 

 cannot be detailed here. In some cases, there are large radiant 

 prisms, all polarizing a golden yellow, and the black cross becomes 

 hardly visible. In others, its divergence is so great, that the yellow 

 sectors assume the appearance of a cross. 



In some still more complex crystals, there is seen one or more 

 narrow black rings, which arise from the absence of matter where 

 they appear. 



2. SaUcine. — This substance yields splendid discs. When of the 

 diameter of g^th to |th of an inch, these tints are of the first and 

 second orders, and they form objects of singular splendour. Here 

 also, the smaller crystals polarize a bluish white. The discs of salicine 



