DOUBLE REFKACTION OF ICELAND SPAR. 
75 
known that all crystals whose three axes are not at right angles 
with each other, give double images of objects seen through 
them in some directions. This property is called double re- 
fraction. 
A great variety of substances not crystalline, possess the 
power of double refraction, — animal substances, such as hairs, 
horns, shells, bones, muscles, nerves, and other tissues ; vegeta- 
ble substances, like certain seeds, starch, gums, resins, essential 
oils and sugar in a fluid state, and many artificial substances, 
as glass unequally tempered. In many of these substances the 
separation of the two images is so slight that the double refrac- 
tion is not ordinarily perceived. The peculiar character of 
such substances will be better understood in connection with 
the sections on Par'tial^ and Colored Polarization. 
The actual separation of the two images produced by double 
refraction, varies greatly in different substances ; to show the 
effect, therefore, a considerable thickness of the substance is 
generally required : transparent crystals afford the best illus- 
trations of this peculiar property. 
100. Double refraction of Iceland Spar. Iceland spar, 
otherwise known as crystallized carbonate of lime^ calcareous 
spar, or calcite, exhibits in a beautiful manner the phenom- 
ena of double refraction. It crystallizes in the form of oblique 
rhombic prisms, as shown in Fig. 39. 39. 
It is bounded by six equal faces, all 
rhombs, meeting each other at angles 
of 105° 5', or its supplement 71° 55'. 
It has three axes, one of which, called 
the vertical or major axis, a b, Fig. 39, 
is equally inclined to each of its six 
faces, at an angle of 45° 23'. A plane 
a cb d, joining two obtuse lateral edges, is called its principal 
section. This crystal cleaves perfectly parallel to either of its 
six natural faces. 
This mineral is very transparent, and in pure specimens 
quite colorless and free from fractures. Any object, as a line 
or point, seen through any of the faces of this crystal, in any 
CATALOGUE OF ACHROMATIC MICROSCOPES. 
