274 Dr. Brewster on the multiplication of images, and the 
which are formed by reflections from the sides of the rhom- 
boid. 
All the specimens of interrupted calcareous spar which I 
have examined, present a very remarkable and beautiful phe- 
nomenon which has not hitherto been observed. Let AEBF, 
Fig. 3. be a section of the rhomboid shown in Fig. 7, and 
BA the interrupting plane ; a ray RS incident in a direction 
nearly parallel to BA will be refracted in the direction ST, 
and passing through the plane BA at T, will emerge in the 
direction VX. Another ray rs incident on the adjacent face BF, 
and parallel to SR, will be refracted in the direction sT, and 
suffering reflection at T in the direction TV will also emerge 
in the line VX. If the face BF is covered, an eye placed at 
X will perceive across the middle image A b, Fig. 2. a series of 
beautiful fringes concave towards B, Fig. 3. and separating a 
bright from a dark space which is towards B. The predo- 
minant colour in these fringes is yellow’ : their direction is per- 
pendicular to the line joining the images a, B , Fig. 2, and they 
increase in breadth towards E, Fig. 3. fading away in pink and 
green fringes. If the face BE is now covered, an eye at X 
will perceive a series of fringes complementary to the first 
set, and having a bright blue for their predominant colour. 
They have the same curvature and direction as the first set, 
and separate a bright from a dark space which is towards E, 
but they are far superior to them in distinctness and splen- 
dour of colouring. By covering only a part of EB, we can 
see at the same time both the sets of fringes, the yellow 
stripes of the first set joining the blue stripes of the second. 
This junction of the fringes forms a very interesting pheno- 
