21 
by doubly refracting crystals. 
also their system of coloured rings, by exposure to common 
light. The hght is, in these cases, analyzed in passing ob- 
liquely through the laminae, in the same manner as if it had 
been transmitted through a bundle of glass plates. 
I shall now conclude this section with a particular account 
of some very interesting phenomena exhibited by several 
crystals in the preceding tables. 
i. Super-acetate of copper. When a prism of this metallic 
salt is exposed to the solar rays, so that the plane of refrac- 
tion is perpendicular to the axis of the rhomboidal prism, and 
the ray passes through the angle of the rhomboid, which is 
70°, two distinct images of the sun will be observed ; and the 
one which has suffered the greatest refraction will be greenish 
yellow, while the other will be of a deep blue colour. This 
separation of the two tints is more distinct in some prisms 
than in others, owing to the manner in which they are cut 
from the rhomboidal crystal, and in certain points of inci- 
dence the two images have the same tint. When a plate of 
super-acetate of copper is ground so thin as to be transpa- 
rent, it has a brilliant green colour, composed of blue and 
greenish yellow. If it is exposed to polarised light, so that 
the axis of the rhomboidal prism is in the plane of primitive 
polarisation, the greenish yellow light will be entirely absorbed, 
and the transmitted ray will be of a deep blue colour. By 
turning the plate round the polarised ray, the greenish yel- 
low rays will re-appear, and will gradually regain their 
former intensity ; while the blue rays will in the same pro- 
portion be absorbed, till after one-fourth of a revolution the 
transmitted light will be wholly greenish yellow. When the 
faces of the plate are perpendicular to any of the two resul- 
