76 POLARIZED LIGHT AND ITS APPLICATION TO THE MICROSCOPE. 
direction except in a line parallel to a 5, will appear double, 
owing to the subdivision of the pencil into two beams. As the 
imaginary line or axis a is that in which no doubling of the 
image takes place, it might with propriety be called the axis of 
NO double refraction.^ but as the amount of separation of the 
two rays depends on their position with reference to a Z>, that 
line is termed the axis of double refraction.^ on all sides of 
which double refraction takes place. This line is fixed only in 
direction ; every other line parallel to a J, is equally a line of no 
double refraction. The other parts of the crystal can be split 
off by natural cleavage so as to leave any line parallel to a b, 
the perpendicular or major axis of the remaining crystal. 
Figure 40 shows the appearance of lines A B, C D, E F, 
G H, and a circle drawn around their common intersection, 
seen through a fiat prism of Iceland spar, about an inch and a 
Fig. 40. quarter thick. A B is 
parallel to the principal 
section of the prism ex- 
pressed hj a G b d, Fig. 
39. The circle and lines 
are all seen in their true 
position when viewed in 
a direction perpendicular 
to the face of the prism, 
but just above the lines, 
and parallel to them, are 
seen with equal bright- 
ness, the dotted lines c d, 
efgh, and about their 
common intersection a second circle, the double image of 
the first. 
The pencil of light is divided into the ordinary and extra- 
ordinary rays. The extraordinary image of a line, seen 
through a double image prism, is always parallel to the ordi- 
nary image. The extraordinary image of the circle shows 
clearly that every point is displaced in a plane parallel to A B, 
or the principal section joining the obtuse lateral edges of the 
prism. Every line drawn parallel to A B, will appear single, 
J. & W. GRUNOW & GO’S ILLUSTRATED 
