22 
ON  IODO- SULPHATE  OF  QUININE. 
rotated  90°,  the  position  of  the  crystals  remaining  the  samer  the 
appearance  of  the  crystals  is  then  reversed ;  that  "which  was 
Fig.  5,  FlG-  6- 
Fig.  7. 
transparent  becomes  opaque,  and  the  opaque  one  diaphanous,  as 
in  fig.  6. 
The  depolarizing  effect  of  the  crystals  is  best  seen  by  using 
one  tourmaline,  placing  a  crystal  of  iodide  across  its  axis,  so  that 
it  shall  become  opaque,  and  then  twQ  crystals  of  the  iodide  across 
this  at  an  angle  of  45°,  so  that 
their  opposite  ends  shall  cross  each 
other  at  90°  at  a  distance  from  the 
opaque  crystal,  as  in  fig.  7,  a,  b,  ca 
The  crystals  a  and  6'  being  in  the  de- 
polarizing angle,  exhibit  this  pheno- 
menon, except  where  they  cross  each 
other  at  right  angles,  at  which  point 
complete  polarization  is  effected,  as 
is  seen  by  the  black  square.  Where 
a  and  b  cross  the  polarized  field  of  e,  depolarization  takes  plaee? 
and  two  rhomboidal  spaces,  pink  and  green,  appear. 
When  two  tourmalines  are  employed,  the  results  are  more  com- 
Fig.  8.  Fig.  9. 
plicated ;  either  both  crystals  become  opaque,  as  a     fig.  8,  or 
