272 Dr. Brewster on the multiplication of images , and the 
light is transmitted through the faces BCGE and AFHD, or 
through the faces BFHC and ADGE, the object from which 
it proceeds has the appearance represented in Fig. 2., con- 
sisting apparently of three images. The middle image A is 
white and is composed of two images A, b polarised in an 
opposite manner like the double image formed by common 
rhomboids of calcareous spar. The image B, which is highly 
and uniformly coloured, is polarised like A, and the image a 
is coloured in the same manner as B, and polarised in an 
opposite manner like the other image at b. 
Let the rhomboid be now placed in such a position, 
that a horizontal pencil of light is incident upon the ver- 
tical face BCGE, and let the rhomboid be turned round a 
vertical axis, so that the pencil may be incident at various 
angles, the plane of incidence being always parallel to the 
horizon. 
When the angle of incidence is about io°, and the ray in- 
clined towards EG, the two images B, a vanish, their angular 
distance being then about 3^-°, but at every other angle of inci- 
dence, these two images are visible. When the angle of inci- 
dence is gradually diminished, till it vanishes and then increases 
on the other side of the perpendicular, the reappearing images 
B, a separate from the middle image. The image B separates 
from it more rapidly, and increases in magnitude, in the same 
manner as when a pencil of light is incident obliquely towards 
the refracting angle of a prism, while the image a separates 
slowly from A, and contracts its dimensions, as when a pencil 
of light is incident obliquely towards the base of a prism. When 
the angle of incidence increases from the position where B 
and a vanish, these images approach to the middle image 
