278 Dr. Brewster on the multiplication of images , and the 
which, such as rand s, are not complete. When any of these 
minute veins, such as mn, is seen through the faces eE Gg, 
bE Gc, Fig. 9, it is quadrupled, and appears, as in the figure, 
composed of four veins mi, m 2, m3, ^4, and ni,n2,n$, n^ 
diverging from m and n. By gently inclining the rhomboid, 
all these veins are brilliantly coloured, and, what is very sin- 
gular, the colours of the middle veins m 2, m3, n2, H3, are 
always complementary to the colours of the extreme veins 
mi,m^,m, n 4, exhibiting a much greater variety of hues 
than is seen in any other position of the crystal. 
In order to observe the connection between the stratum eh 
and the contiguous prisms, I cut off part of the prism e E b and 
laid bare the surface of the stratum towards E. I then re- 
moved the stratum itself till I came to the adjacent surface of 
the prism, and in both cases I found the particles of the 
prisms adhering firmly to the stratum, though they were at 
such a distance from it that light incident obliquely suffered 
reflection. 
From these experiments, we may safely conclude, that the 
interrupting stratum is not a fissure or fracture ; — that it is a 
crystallized vein of calcareous spar, cohering firmly to the 
adjacent mass ; — and that the multiplication of images and 
the colours which accompany them, are produced only when 
this vein is interposed between two solid prisms. 
Sect. III. On the cause of the multiplication of the images. 
If a ray of light RS is incident upon a rhomboid EBFA, 
Fig. 4, interrupted by a stratum of air AB, it will be divided 
by refraction into two pencils Sa, S b. These pencils will be 
again refracted at the second surface m n into the directions 
