276 Dr. Brewster on the multiplication of images, and the 
breach of continuity between the two prisms. If the adjacent 
surfaces of the prisms were perfectly smooth, and flat, like 
plates of parallel glass, and if they were pressed together 
by a great force, total reflection would thus be prevented, and 
the light would pass through the fissure at any obliquity. 
But if total reflection were prevented in this manner, the pencil 
of light would experience no peculiar action in passing through 
the compressed surfaces, and therefore neither a multiplica- 
tion of images, nor a decomposition of the pencil into colours 
could take place. 
With the view of corroborating this reasoning, I endeavoured 
to separate the two prisms by force, but I found this quite 
impracticable. The crystal actually broke at another place, 
so that the two prisms cohere with great force, though this 
is the direction of one of the cleavages of calcareous spar, 
and though the supposed fissure extends to the very surface 
of the four faces of the rhomboid. 
But admitting the existence of a fissure under these cir- 
cumstances, it is demonstrable that it could not produce the 
phenomena described in the preceding section. I have ex- 
amined several real fissures in calcareous spar, and though 
the colours of thin plates were seen by reflection, yet those 
formed by transmitted light, could scarcely be rendered 
visible. 
The appearance on the middle image of colours complemen- 
tary to those on the extreme images, is an irrefragable proof 
that they are not the colours of thin plates, in which one of 
the complementary tints must necessarily suffer reflection. 
In order to remove all doubt respecting the effects of a 
fissure, I ground off the angles EG, FH, Fig. 1. till the 
