colours which accompany them in calcareous spar . 273 
A6; constantly varying their colours in such a manner, that 
the middle image A6 has a colour complementary to that of the 
extreme images B and a. 
If the angle of incidence is made to vary in a vertical plane, 
the extreme images separate from the middle image when 
the pencil is inclined to Gc, but they approach it, and exhibit 
the complementary colours when the inclination of the pencil 
is towards BE. During these changes the colour of A is 
often complementary to that of b. 
When the eye is placed in front of the face BCGE, so as 
to perceive the images reflected from the posterior surface 
AFHD, each of the images a, b, A, B is tripled in passing the 
plane ABCD, so that nine highly coloured images are dis- 
tinctly visible. 
In a very curious specimen of calcareous spar in the pos- 
session of Mr. Mylne, there are two supposed fissures as 
shown at afhd and ebcg, fig. 7. These planes are equidistant 
from the obtuse angles E, F, and each of them produces 
three images which are never coloured, except when the 
incident ray is very oblique, and the two extreme images near 
the middle one. In this case, the colour of the middle image 
is distinctly complementary to that of the extreme images. 
When the light passes through the faces AEGD, BFHC in 
such a manner, that the eye receives part of the pencil acted 
upon by the plane ebcg, and part of the pencil acted upon 
by the plane af h d each of the three images appears dou- 
ble, and in consequence of a third plane e/B> cy, one of the 
extreme images is sometimes tripled, so that the eye may 
see at once seven images independent of the numerous images 
mdcccxv. N n 
