ILLUMINATION OF OBJECTS; POLARIZED LIGHT 51 



through them, no matter what the direction may be; such bodies 

 are called isotrojncjind exhibit but a single index oj refraction; 

 ether waves proceeding from any pokffare^spEerical ; (2) they 

 are not optically homogeneous, but transmit light waves with 

 different velocities in different directions; in this case they are 

 called ceolotropic or anisotropic; ether waves proceeding from a 

 point are ellipsoidal. In the first class are found the so-called 

 amorphous bodies and substances crystallizing in the isometric 

 or cubic system, 1 while in class 2, we find substances crystallizing 

 in the hexagonal, tetragonal, orthorhombic, monoclinic and triclinic 

 systems, and occasionally bodies normally iso tropic 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 iso tropic or anisotropic. 



To study the optical behavior of tiny crystals or transparen 

 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 2 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. 



