160 ELEMENTARY CHEMICAL MICROSCOPY 



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 tri- 

 clinic 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 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. 



A nicol prism consists of a long rhomb of calcite cut length- 

 wise in an oblique plane forming angles of 90 degrees with the 

 upper and lower faces of the rhombs and cemented together again 

 with Canada balsam, see Fig. 101. If a ray of light R enters 

 such a prism it is polarized, being resolved into two component 

 rays vibrating at right angles to each other. One of these rays 

 O, known as the ordinary ray is deflected slightly more than the 



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. 



