AmAprn"-i?9brm"}     Polarization  without  a  Polarizer.  183 
years  ago,  I  tried  to  make  use  of  this  then  well-known  property  of 
the  atmosphere,  but  soon  gave  it  up  for  reason  of  the  loss  of  time 
involved  by  the  continual  readjusting  of  the  position  of  the  instru- 
ment, and  especially  by  the  want  of  sufficient  sky  surface  to  which 
most  residents  of  large  crowded  cities  are  subject.  I  therefore 
devised  in  its  place  a  very  cheap,  steady  and  reliable  polarizer,  con- 
sisting of  a  piece  of  plate  glass,  ground  and  blackened  at  the  back 
surface,  and  attached  under  the  stage  at  the  angle  for  maximum 
polarization  which  for  reflection  from  plate  glass  is  56^°.  This 
device  is  equally  good  by  night  or  day,  clear  or  clouded  sky,  the 
only  precaution  is  to  place  the  microscope  in  such  a  position  that 
thq  light  reflected  by  this  fixed  mirror  is  directed  through  the  axis 
of  the  tube.  It  is  as  good  a  polarizer  as  a  large  Nicol  prism,  and 
far  better  and  more  available  than  the  blue  sky. 
Hagenbach  observed  that  not  only  the  light  of  the  blue  sky  is 
polarized,  but  also  that  light  which  the  sun  sends  through  such 
layers  of  air  as  are  between  distant  mountains  and  our  eyes.  The 
polarization  is  especially  strong  when  the  background  is  dark  and  the 
intervening  layers  of  air  not  too  small.  When  the  distant  moun- 
tains become  indistinct  by  the  action  of  the  reflected  light  of  the 
intervening  layers  of  air,  the  mountain  may  again  be  very  distinctly 
seen,  when  a  Nicol  prism  or  analyzer  is  placed  before  the  eye  piece 
of  the  telescope.  He  tested  this  by  looking  during  a  clear  day  at 
the  Alps  from  such  a  great  distance  that  they  could  no  more  be 
distinctly  seen,  even  by  a  telescope  ;  they  became  at  once  clearly 
and  sharply  defined,  when  he  placed  a  Nicol  prism  before  the  eye 
piece,  or  even  before  the  naked  eye,  rotating  the  prism  until  the 
proper  position  was  obtained. 
It  is  unfortunate  that  in  clouds  and  fogs  no  polarization  takes 
place,  otherwise  a  Nicol  prism  would  enable  us  to  make  our  vision 
penetrate  in  certain  directions.  Experiments  have  been  made,  but, 
of  course,  without  success. 
Polarization  of  Pure  Water. — It  was  suspected  that  water,  when 
pure  enough,  would  polarize  the  light  according-  to  the  same  laws 
as  the  pure  blue  atmosphere ;  this  was  found  to  be  actually  the  case 
by  Soret,  who  experimented  in  the  pure  blue  waters  of  the  Swiss 
lakes,  using  for  that  purpose  a  tube,  with  a  piece  of  plate-glass 
fixed  water-tight  on  the  lower  end,  while  the  upper  end  car- 
ried a  Nicol  prism.     It  is  essential  in  this  experiment  that  the 
