78 POLARIZED LIGHT AND ITS APPLICATION TO THE MICROSCOPE. 
illumination. This prism is constructed from an elongated 
rhombohedron of Iceland spar in the following manner : The 
natural faceP of the prism, Fig. 41, which makes, with the ob- 
tuse lateral edge K, an angle of about 71°, is 
ground away so as to form a new face, 
making an angle of 68° with the edge K, 
and a right angle with the plane of princi- 
pal section joining the obtuse lateral edges 
K and IP, which is the same as the section 
a G I Fig. 39. From the obtuse solid 
angle E, the prism is sawn through in the 
direction E F, making a right angle with 
the new terminal face, and also with the 
plane of principal section. From F another 
terminal face is constructed at right angles 
with the section E F and with the principal 
section, and making an angle of 68° with the edge K'.* All 
the new faces are now carefully polished, and the two parts of 
the prism are cemented together, in their former 
position, with Canada balsam. The lateral faces 
of this compound prism are all painted black, 
leaving only the terminal faces for the transmission 
of light. 
Figure 42 represents a section of Nicol's prism 
through the obtuse lateral edges, and shows the 
course of the two polarized rays into which com- 
mon light is divided by this prism. A ray of com- 
mon light, a h, entering this prism, is refracted 
into the ordinary ray h c, and the extraordinary ray 
h d. The index of refraction of Iceland spar, for 
the ordinary ray, being 1.6543, and that of balsam 
only 1.536, the ordinary ray suffers total reflection 
at the surface of the balsam, and cannot pass into 
the lower part of the prism, unless the incident 
* In tlie manufacture of Nicol’s prisms, the inclination of the terminal faces is 
sometimes varied to suit particular purposes. 
Fig. 42. 
Fig. 41. 
J. & W. GRUNOAV & CO’S ILLUSTRATED 
