14,8 Dr. Brewster on the action of 
medium, arranged in alternate strata. When the superficial 
polish of both these media is removed, the application of the 
halsam restores, as it were, the polish of the ordinary medium, 
without restoring that of the extraordinary medium ; while 
the application of the alcohol restores the polish of the extra- 
ordinary medium without restoring that of the ordinary 
medium. 
When the refractive power of the fluid is intermediate 
between that of the two media, the ordinary and the extra- 
ordinary image will be equally indistinct ; and we have it in 
our power to alter the distinctness of either of the images, by 
varying the refractive force of the interposed fluid. 
If the plane of incidence is not perpendicular to the axis,, 
a variation in the angle of incidence will produce a variation 
in the index of refraction of the extraordinary ray ; and, since 
the refractive power of the interposed fluid suffers no change, 
the extraordinary image must become more or less distinct, 
according as its index of refraction is made to approach more 
or less to that of the fluid, by varying the inclination ol the 
refracted ray to the axis. 
From the preceding experiments, which have been repeated 
with the same results with calcareous spar , arragonite , and many 
other crystals, we may deduce the following conclusions : 
1. The force of double refraction and polarisation extends 
not only without the interior limit of the ordinary refractive 
force, but also without the surface of the crystal. 
2. The force of double refraction and polarisation emanates 
from the surface of bodies, though its intensity depends upon 
the inclination of the surface to the axis of the crystal. 
g. The ordinary or the extraordinary image may be ex- 
