146 ANNUAL OF SCIENTIFIC DISCOVERY. 



with the small leus, but all confused in that with the surrounding 

 apertures. 



ON THE OPTICAL PKOPERTIES OF A RECENTLY DISCOVERED SALT 



OF QUININE. 



THE above is the title of a paper read before the British Associa- 

 tion, Belfast, by Prof. Stokes. The salt referred to is stated by Dr. 

 Herapath, to be easily formed by dissolving bi-sulphate of quinine in 

 warm acetic acid, adding a few drops of a solution of iodine in alco- 

 hol, and allowing the liquid to cool ; when the salt crystallizes in thin 

 scales, reflecting (while immersed in the fluid,) a green light with a 

 metallic lustre. When taken out of the fluid the crystals are yellow- 

 ish green by reflected light, with a metallic aspect. The following 

 observations were made with small crystals formed in this manner : 

 The crystals possess in an eminent degree the property of polariz- 

 ing light, so that Dr. Herapath proposed to employ them instead of 

 tourmalines, for which they would form an admirable substitute, could 

 they be obtained in sufficient size. They appear to belong to the pris- 

 matic system ; at any rate, they are symmetrical (so far as relates to 

 their optical properties and to the directions of their lateral faces,) 

 with respect to two rectangular planes perpendicular to the scales. 

 These planes will here be called respectively the principal plane of 

 the length, and the principal plane of the breadth, the crystals being 

 usually longest in the direction of the former plane. When the crys- 

 tals are viewed by light directly transmitted, which is either polarized 

 before incidence or analyzed after transmission, so as to retain only 

 light polarized in one of the principal planes, it is found that with 

 respect to light polarized in the principal plane of the length the crys- 

 tals are transparent and nearly colorless, at least when they are as 

 thin as those which are usually formed by the method above men- 

 tioned But with respect to light polarized in the principal plane of 

 the breadth, the thicker crystals are perfectly black, the thinner ones 

 only transmitting light, which is of a deep red color. When the crys- 

 tals are examined by light reflected at the smallest angle with which 

 the observation is practicable, and the reflected light is analyzed, so 

 as to retain, - - first, light polarized in the principal plane of the length, 

 and secondly, light polarized in the other principal plane, -- it is found 

 that in the first case the crystals have a vitreous lustre, and the 

 reflected light is colorless, while in the second case the light is yellow- 

 ish green, and the crystals have a metallic lustre. When the plane of 

 incidence is the principal plane of the length, and the angle of inci- 

 dence is increased from to 90, the part of the reflected pencil 

 which is polarized in the plane of incidence undergoes no remarkable 

 change, except perhaps that the lustre becomes somewhat metallic. 

 When the part which is polarized in a plane perpendicular to the 

 former is examined, it is found that the crystals have angle of polari- 

 zation, the reflected light never vanishing, but only changing its color, 

 passing from yellowish green, which it was at first, to a deep steel 



