108 Proceedings of the Royal Physical Society. 
tion of light was not effected by the whole surface of the sub- 
stance, but only by inclined parts of it which intersected the 
ray in an oblique direction. In passing through such parts a 
beam of light was divided into two rays polarized in opposite 
planes. The one ray continued its original course, while the 
other suffered deflection proportionate to the inclination of the 
polarizing surface, and was thus entirely thrown out of the 
field of view. It occurred to the author that large crystals, 
having their surface artificially inclined, might, when im- 
mersed in a highly refractive fluid, be used as polarizing 
prisms, and he had succeeded in thus forming polarizers, one 
of which he placed before the Society, and which was found 
to be well adapted for application to the microscope. It con- 
sisted simply of a wedge of nitrate of potash, cut from one of 
the angles of a large crystal, and put up in a cell filled with 
castor oil. 
Dr Wright stated that very beautiful objects for the polari- 
scope might be formed of clear plates cut from the surfaces of 
large crystals of nitre, and grooved in various designs. These, 
mounted on a colouring plate of mica, and enclosed in a plate- 
glass cell filled with oil of turpentine, gave rich effects of 
changing tint when traversed by light reflected from a glass 
plate or from the northern sky» Large planes of parallel 
polarizing cr^^stals might be produced by crystallizing muriate 
of morphia on slightly inclined plates of glass, and might be 
removed so as to leave designs. These figures, when mounted 
in Canada balsam, with colouring plates of mica, gave curiously 
dazzling binocular effects, or when viewed with both eyes, as 
the two images differed in colour for each eye. In examining 
objects of this class, it was necessary to stop off all rays but 
those passing at right angles to the polarising plate. 
VI. Notes on the Geology of Swellendam, South Africa. By William 
Carruthers, Esq. (Specimens of the rocks and fossils were ex- 
hibited.) 
The specimens which formed the subject of this notice were 
gathered and sent to this country by Mr Robert Douglas, a 
.solf'-cducated and enthusiastic geologist. They were intended. 
