

Pe PONS Ph SP, 
- TRANSACTIONS OF THE SECTIONS. 15 
By Professor Stoxes, M.A, F.R.S 
_ This salt is desctibed by Dr. Herapath in the Philosophical Magazine for March 1852, 
and is easily formed in the way there recommended, namely, by dissolving disul- 
phate of quinine in warm acetic acid, adding a few drops of a solution of iodine in 
alcohol, and allowing the liquid to cool, when the salt crystallizes in thin scales re- 
flecting (while immersed in the fluid) a green light with a metallic lustre. When 
taken out of the fluid the crystals are yellowish-green by reflected light, with a me- 
tallic aspect. The following observations were made with small crystals formed in 
this manner; and an oral account of them was given at a meeting of the Cambridge 
Philosophical Society, shortly after the appearance of Dr. Herapath’s paper. 
The crystals possess in an eminent degree the property of polarizing 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 prismatic system; at any rate they are symmetrical (so far as re- 
lates to their optical properties and to the directions of their lateral faces) with re- 
spect 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 crystals are viewed by light directly transmitted, which is either polar- 
ized before iticidence or analysed after transmission, so as to retain only light polar- 
ized in one of the principal planes, it is found that with respect to light polarized 
in the principal plane of the length the crystals are transparent, and nearly colour- 
less, at least when they are as thin as those which are usually formed by the method 
above mentioned. 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 colour. 
When the crystals are examined by light reflected at the smallest angle with which 
the observation is practicable, and the reflected light is analysed, 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 colourless; while in the second case the 
light is yellowish-green, and the crystals have a metallic lustre. When the plane 
of incidence is the principal plane of the length, and the angle of incidence is in- 
creased from 0° 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 perpen- 
dicular to the former is examined, it is found that the crystals have no angle of 
polarization, the reflected light never vanishing, but only changing its colour, passing 
from yellowish-green, which it was at first, to a deep steel-blue, which colour it 
assumes at a considerable angle of incidence. When the light reflected in the prin- 
cipal plane of the breadth is examined in a similar manner, the pencil which is 
polarized in the plane of incidence undergoes no remarkable change, continuing to 
have the appearance of being reflected from a metal, while the other or colourless 
pencil vanishes at a certain angle, and afterwards reappears, sq that in this plane 
the crystals have a polarizing angle. 
If, then, for distinction’s sake, we call the two pencils which the crystals, as belong- 
ing to a doubly refracting medium, transmit independently of each other, ordinary 
and eatraordinary, the former being that which is transmitted with little loss, we 
may say, speaking approximately, that the medium is transparent with respect to 
the ordinary ray and opake with respect to the extraordinary, while, as regards 
reflexion, the crystals have the properties of a transparent medium or of a metal, 
according as the refracted ray is the ordinary or the extraordinary. If common light 
merely be used, both refracted pencils are produced, and the corresponding reflected 
pencils are viewed together; but by analysing the reflected light by means of a 
Nicol’s prism, the reflected pencils may be viewed separately, at least when the ob- 
servations are confined to the principal planes. The crystals are no doubt biaxal, 
and. the pencils here called ordinary and extraordinary are those which in the lan- 
guage of theory correspond to different sheets of the wave sutface. The reflecting 
properties of the crystals may be embraced in one view by regarding the medium as 
On the Optical Properties of a recently discovered Salt of Quinine. - 
