8 
sorby: optical characters of minerals. 
1*43 in fluor to 2*34 in blende. If, however, the crystal 
possesses strong double refraction, the phenomena are far 
more complex, and vary according to the direction in which 
the section is cut, its azimuth to the lines of the grating, and 
also according as it has one or two optic axes. 
If we look through a parallel plate of a uniaxial crystal 
with powerful double refraction, like calcite, cut perpendicular 
to the principal axis, we see two undistorted images of the 
circular hole, directly superimposed, one over the other, but 
separated vertically by a wide interval. This doubling of the 
image is due to the collection by the object-glass of divergent 
light, since, for strictly parallel rays passing in the same 
general direction, there is no double refraction whatever. 
Both systems of perpendicular lines are seen in focus at the 
same time in each of the images, one of which is due to the 
ordinary, and the other to the extraordinary, ray. By observ- 
ing these focal points we obtain two indices of refraction, one 
being the true index of the ordinary ray (/a), and the other 
not that of the extraordinary ray (//), but a very low apparent 
index, equal to 
When the section is cut in other directions the images 
differ very much from one another. That due to the ordinary 
ray has invariably the same properties. The circular hole is 
not distorted, and both systems of line are in focus at the 
same time, so that we may call that image unifocal The 
other image, due to the extraordinary ray, instead of thus 
maintaining a constant character, changes very greatly, the 
maximum of change being when the section is cut parallel to 
the principal axis of the crystal. There is no focal point 
whatever at which the circular hole is seen of its true size and 
shape, and the entire circumference is never all in focus at 
once. There are two special foci, widely separated, at which 
the circle is, as it were, drawn out into a long band, at one 
