ON  IODO- SULPHATE  OF  QUININE. 
21 
Fig.  2 
they  cross,  a  deep  chestnut  brown.  If,  however,  the  crystals  are 
placed  on  each  other  with  their  axes  parallel,  no  effect  is  pro- 
duced. This  is  illustrated  by  fig.  2,  where 
the  two  hexagonal  prisms  overlap  verti- 
cally, and  the  full  polarizing  effect  is  visi- 
ble where  the  third  crystal  overlaps  ths 
vertical  one  transversely;  when  three 
crystals  are  examined,  two  at  right  an- 
gles and  a  third  crossing  these  at  45°, 
and  between  them,  as  at  figure  3,  the  in- 
terposed erystal  permits  the  light  to  pass 
through  at  its  line  of  contact  with  the 
others,  and  the  phenomena  of  depolarization  is  produced,  just  as 
when  the  selenite  plate  is  introduced  between  the  tourmalines. 
In  fig.  3  the  prisms  a  and  b  cross  at 
the  square  d,  and  produce  a  perfectly 
dark  field ;  the  prism  c  crosses  this  at  an 
angle  of  45°  ;  the  hexagonal  space  is  of 
course  bright  pink  from  the  depolarizing 
effect  described ;  the  four  small  triangular 
spots  are  somewhat  darkened,  in  conse- 
quence of  polarization  occurring  from 
the  reciprocal  action  of  c  on  a  and  b. 
Fig.  3. 
Fig.  4 
In  figure  4,  the  three  hexagonal  plates  exhibit  the  same  phe- 
nomena ;  a  and  b  cross  each  other  at  90°, 
whilst  the  plate  c  intersects  them  at  45°, 
passing  between  them ;  the  half  crystal 
d  pentagonal  in  form,  is  the  polarized 
space,  and  No.  2,  where  c  interposes,  pro- 
ducing pink  as  before  ;  spaces  1  and  3  are 
partially  polarized  and  darkened. 
When  a  pair  of  these  crystals  at  right 
angles  are  viewed  with  a  single  tourma- 
line, or  Nichol's  prism,  the  crystal,  whose 
axis  is  transverse  that  of  the  tourmaline,  is  black  as  night,  whilst 
the  other,  whose  axis  is  parallel  with  that  of  the  tourmaline,  is 
perfectly  transparent,  as  in  fig.  5.    If  the  tourmaline  is  now 
