Plates  of  tourmaline  made  by  vertical  sections  of  a
transparent  crystal  transmit  the  extraordinary  ray,  while
the  ordinary  ray  is  absorbed.  Hence  such  plates  are  con-
venient for  optical  investigations.  A  simple  polariscope
made  of  two  tourmaline  plates  has  the  form  in  Fig.  3.
The  effects  are  the  same  whichever  tourmaline  plate  is
brought  to  the  eye.  The  plate  away  from  the  eye,  or  that
receiving  the  light  for  transmission,  is  called  ih&  polarizer.
ra  PUT81CAL  PB0PEBTTE8   OF  HIITEBALS.
snd  the  other  the  analyzer.  Light  passes  freely  through
the  two  plates  as  long  as  they  have  the  position  they  had  m
the  cryatal,  that  ia,  hare  the  rertical  axes — ^the  pkiieB  of
Tibration — of  the  two  parallel.  Bnt  if  the  axes  are  crossed,
by  revolving  one  plate  90°,  no  light  passes.     In  a  revolu-


tion, light  and  dark  fields  alternate  every  90°.  Crystalline
minerals  are  examined  by  placing  sections  ot  them  between
the  tourmalines.
Calcite,  owing  to  the  wide  divergence  of  its  refracted
lays,  is  commonly  nsed  for  polarizing  apparatas.     In  a
4.  "nicol  prism"  of  calcite  (Fk;.  4)  the  extraor-

dinary ray  {ac')  passes  through  the  prism,  while
the  other  {ac)  is  got  rid  of  by  reflection  from
the  snrFace  of  Canada  balsam  {mn)  along
which  the  two  pieces  of  calcite  in  the  prism  are
joined.
