ILLUMINATION OF OBJECTS; POLARIZED LIGHT 
51 
through them, no matter what the direction may be; such bodies 
are called isotropic and exhibit but a single index of refraction; 
ether waves proceeding from any point are spherical; (2) they 
are not optically homogeneous, but transmit light waves with 
different velocities in different directions; in this case they are 
called (Eolotropic or anisotropic; ether waves proceeding from a 
point are elhpsoidal. In the first class are found the so-called 
amorphous bodies and substances crystallizing in the isometric 
or cubic system,^ while in class 2, we find substances crystallizing 
in the hexagonal, tetragonal, orthorhombic, monoclinic and triclinic 
systems, and occasionally bodies normally isotropic but which 
under certain stresses and strains lose their homogeneity in one 
or more directions. If instead of employing ordinary light in 
which the ether vibrations are in all possible azimuths and where 
the paths of vibration of the ether particles are constantly 
changing, we illuminate the objects with plane polarized light 
in which the ether vibrations are parallel to a single plane it 
becomes much easier to ascertain whether the transparent object 
is isotropic or anisotropic. 
To study the optical behavior of tiny crystals or transparent 
bodies, use is made of the polarizing microscope. For ordinary 
chemical investigation the polarizing apparatus may be quite 
simple, but in crystallographic and petrological studies elaborate 
and most carefully constructed and adjusted instruments are 
essential; with this latter type of instrument ^ the chemist 
rarely has anything to do. 
The polarizing apparatus of the commonly employed chemi¬ 
cal microscopes usually consists of two nicol prisms, one placed 
below the stage, the other above the microscope objective. 
1 Certain crystals belonging to the isometric system behave in a similar manner 
to optically active chemical compounds in solution, in that they possess the power 
of rotating the plane of polarization of light sent through them, either to the right 
or to the left, independently of the direction of transmission. Such anomalous 
crystals, although isotropic, may be said to be doubly refractive. This phenomenon 
is termed circular polarization. 
2 For a very comprehensive discussion of the Petrological Microscope, see F. E. 
Wright, Pub. No. 158 of the Carnegie Institution of Washington, The Methods 
of Petrographic-Microscopic Research. 
