colours which accompany them in calcareous spar. 279 
ac, bd, and falling upon the second prism at c, d, each of 
them will be again divided into two pencils, viz. ac into the 
pencils ce, cj\ and bd into the pencils dh , dg. The pencils 
cf, dg emerging parallel to each other, will form a double 
image like A b, Fig. 2, while the other two pencils ce, dh will 
be inclined to these, and will form the single images a , B. 
The images will therefore be multiplied exactly as in Fig. 2, 
and if we calculate their angular distance, we shall find it 
coincident with the experime'ntal results. This multiplication 
of the images may perhaps be more easily comprehended by 
supposing A, B, Fig. 2. to be two images formed by the first 
prism ABE, Fig. 4; then as the second prism ABF has an 
equal refracting angle, but placed in an opposite direction, it 
will refract the image B to b, and the image A to a, thus form- 
ing a double image in the middle, and a single image on each 
side of it polarised in the manner described in Sect. I. 
In the preceding reasoning it is assumed, that there is an 
interruption in the structure of the rhomboid by which a sub- 
division of the rays takes place within the crystal. We shall 
now enquire how such an effect can be produced without a 
fissure. If we divide a rhomboid into two prisms ABE, ABF, 
and fill up the interval AB with a cement of the same or of 
a different refractive power from that of the calcareous spar, 
the ray RS will emerge in four pencils ce, cf, dg, dh, just as 
when AB was a stratum of air, and in so far as the multipli- 
cation of images is concerned, this artificial rhomboid will 
exhibit the precise phenomena described in Sect. I. 
Hence it follows, that the multiplication of images arises 
from a subdivision of the two pencils at the first surface 
