OPTICAL PI{OPI.I;TII> 



CRYSTALS 



179 



fee 



FIG. 329. Tourmaline Analyzer. 



(op, :iiul lea-1 when :it right angles \n it, showing that some of the 

 reflected light is |)olari/cd ;m<l that the plane of vibration of the 

 polari/ed reflected light i> parallel to the table top and at right 



angles to the plane of iliciileliee. 



In testing the vibration planes of the two rays transmitted by the 

 ealcite rhomb, when the vibration plane of the tourmaline section 

 is parallel to the long diagonal of the rhombic face of the ealcite, 

 Fig. :{'J7, only the ordinary ray 

 will appear; its vibration plane 

 must therefore be parallel to this 

 diameter. Upon revolving the 

 tourmaline section, both rays ap- 

 pear and are equal in intensity 

 alter a revolution of 45, Fig. 328. 

 Upon revolving the tourmaline 

 90 only one ray will appear, the 

 extraordinary ray, the vibration 

 plane of which must therefore be 

 parallel to the short diagonal of 

 the calcite face, Fig. 329. The 

 two rays are polarized and their vibration planes are at right 

 angles ; this is true of all anisotropic substances. 



The nicol prism. As polarized light is necessary in the study 

 of the optical properties of minerals, and to avoid -the natural 

 color of tourmaline sections, Nicol in 1828 devised the instrument 

 now used as the source of polarized light in most optical instru- 

 ments and known as the nicol prism or nicol. 



It is constructed of clear colorless calcite in such a manner that 

 one ray, the ordinary ray, is internally totally reflected and ab- 

 sorbed, while only the extraordinary ray emerges, thus yielding 

 plane polarized light, all of which is vibrating in one known plane. 



Fig. 330 is a section through the short diagonal of a nicol prism, 

 illustrating its construction. A clear, colorless cleavage piece of 

 calcite, three times as long as broad, is cut along the plane PP' per- 

 pendicular to the plane of the diagram, from the obtuse angle at P 

 to that of P'. The angle PP'e should be 22 ; the end surfaces are 

 t hen cut down until the angles dP'P and ePP' are right angles. The 

 two polished halves are cemented together in their original posi- 

 tion with Canada balsam, a film of which will separate the two 

 halves and lie along the plane PP'. The cemented calcite is then 

 set in cork, the walls of which next the calcite have been blackened 



