132 
IOWA ACADEMY OF SCIENCE Voi,. XXIX, 1922 
I 
by mirror M 2 to pass through Nicol N 1 to the opaque illuminator 
of the Saveur type in one of the regular metallographic micro- 
scopes of Bausch and Bomb. This plane polarized light, with the 
electric vector horizontal, was reflected through the objective sys- 
tem (usually an 8 mm. objective was employed), and fell nor- 
mally upon the crystal X. In fact this is really a cone of light 
which falls upon the crystal, but the angle of the cone is only 
about 3°, and so for all practical purposes the incidence is normal. 
The crystal X is mounted on the stage in a special holder and can 
be turned without loss in centering or in maintenance of plane, 
so that its long axis is either parallel or perpendicular to the 
electric vector of the incident plane polarized light. The reflected 
light passes up through the tube, and forms a real image of the 
crystal surface on the interface of the double prism P. This image 
owes its intensity to the magnitude of the reflecting power of 
the crystal in the particular position it happens to occupy. The 
double prism P consists of two small right-angled prisms, one 
of them silvered, with the silver film cut into a grid by re- 
moving alternate narrow strips of the silver. That portion of the 
image passing between the strips is viewed by the eyepiece E. 
The upper portion of the beam from L x passes through the two 
Nicols, W 2 and N s , illuminating the silver strips. The latter are 
viewed by the same eyepiece E. By adjusting the Nicol N 2 (N s set 
to make the electric vector horizontal) a match in intensity can be 
made for each wave length, and for each position of the crystal. 
With this arrangement, as far as described, only the relative re- 
flecting powers in the two principal positions can be determined. 
In order to obtain the absolute reflecting powers a piece of glass, 
2 Sieg, Proc. Ia. Acad. Sci., 23, p. 179, 1916. 
