140  A  Cheap  Substitute  for  the  Selenite.  {Ami^X%™m' 
As  will  be  seen  from  the  quantity  of  oxygen  in  fractions  I  and  9 
the  composition  of  this  oil  is  much  more  complex,  than  was  sup- 
posed.   The  investigation  will  be  continued. 
Pharmaceutical  Laboratory,  University  of  Wisconsin, 
references. 
(1)  Gustave  V.  Kradwell,  Proc.  Wise.  Phartn.  Ass.,  1891,  p.  37. 
(2)  Louis  H.  Kressin,  ibidem,  1892,  p.  66. 
A  CHEAP  SUBSTITUTE  FOR  THE  SELENITE. 
By  Hans  M.  Wieder. 
A  set  of  selenite  plates  (generally  three)  is  an  indispensable 
adjunct  to  the  polarizing  outfit,  because  of  the  beautiful  display  of 
colors  caused  by  their  use. 
Having  on  a  certain  occasion  mislaid  my  selenites,  I  looked  about 
for  a  substitute,  and  bethought  myself  of  trying  what  mica,  which 
is  stated  to  possess  similar  light-retarding  power  as  selenite,  might 
do.  To  my  agreeable  surprise  mica  (the  common  stove-door  kind) 
proved  to  be  fully  equal  to  any  ordinary  selenite,  the  colors  being 
quite  as  handsome.  Mica  possesses  two  advantages:  It  is  much 
cheaper,  a  piece  three  inches  square  costing  about  two  cents, 
while  a  selenite  (the  usual  size  of  which  is  5/§  of  an  inch )  costs 
from  one  dollar  up,  and  mica  can  be  cut  with  scissors  and  handled 
without  much  fear  of  breaking  it.  The  best  way  to  proceed  is  as 
follows  : 
Put  a  slide  of  any  polarizing  substance  (starch  for  instance)  on 
the  stage  of  the  microscope,  having  previously  put  the  two  nicols 
in  their  places,  and  slip  a  piece  of  mica  under  the  slide.  Some 
kind  of  color  will  be  observed.  If  not  satisfactory  or  especially 
handsome,  give  the  mica  a  slight  turn  around  its  axis,  and  try  in 
this  way  whether  in  any  position  a  satisfactory  color  is  obtained. 
When  found,  cut  one  end  square  so  as  to  be  parallel  with  the  slide ; 
by  always  slipping  in  the  mica  in  the  same  way  the  same  color  dis- 
play will  be  obtained,  since  the  retarding  power  and  consequently 
the  color  varies  with  the  thickness.  Quite  a  variety  of  beautiful 
color  effects  may  be  obtained  by  either  using  mica  plates  of  varying 
thickness,  or  by  merely  using  two  or  more  layers  of  thin  plates 
superimposed.    The  colors  may  be  varied  still  more  by  altering  the 
