1 84  Polarization  without  a  Polarizer.  {A%pXiSarnK 
lake  is  so  deep  that  the  bottom  cannot  be  seen,  and  also  that  the 
surface  is  smooth,  as  otherwise  the  solar  rays  do  not  enter  the  water 
parallel.  When  the  sun  does  not  shine  on  the  water  the  polariza- 
tion is  very  imperfect,  as  then  the  light  enters  it  from  many  differ- 
ent sides,  andxeven  when  the  sun  shines  the  polarization  is  imper- 
fect for  the  same  reason,  because  light  enters  it  from  various 
directions,  while  the  blue  atmosphere  is  only  illuminated  by  the 
great  luminary,  the  sun. 
Wheatstone' s  Polar  Clock. — A  very  ingenious  application  of  the 
polarization  of  the  well-defined  regions  of  the  blue  sky  has  been 
made  by  Wheatstone  in  his  so-called  Polar  Clock,  by  which  the 
hour  of  the  day  may  be  ascertained  by  observing  in  the  blue  sky, 
in  the  direction  of  the  celestial  North  Pole,  the  atmospheric  polari- 
zation and  its  amount.  The  instrument  consists  of  a  tube  placed, 
parallel  to  the  terrestrial  axis,  so  as  to  give  it  a  fixed  direction  in 
space  not  influenced  by  the  earth's  rotation.  It  is  fitted  at  one  end 
with  a  double  image  prism,  as  an  eye  piece,  and  has  at  the  other 
end,  directed  to  the  North  Pole,  a  small  hole  covered  with  a  thin 
plate  of  selenite,  of  which,  when  looking  through  the  prism,  two 
images  may  be  seen  of  opposite  color.  This  double  image  prism 
is  capable  of  rotation,  and  carries  an  index,  whi^h  points  the  hour 
marked  on  a  half  circle.  As  the  maximum  plane  of  polarization  is 
always  seen  900  from  the  sun,  the  index  will  point  to  the  right 
hour,  when,  by  rotating  the  prism,  the  position  is  obtained  showing 
no  color  in  the  two  selenite  images  seen.  Of  course,  the  prism 
must  be  fixed  once  for  all  in  the  right  position  by  trial,  and  when 
once  properly  placed,  the  solar  time  may  be  found  without  making 
use  of  the  sun  itself,  and  even  when  the  sun  is  invisible,  either 
behind  a  cloud  or  below  the  horizon,  during  the  twilight  morning 
or  evening ;  then  the  hour  may  be  found  by  the  observation  of  the 
illumination  of  the  atmosphere,  because  the  shifting  polarization 
moves  with  the  sun  and  may  be  observed  when  only  the  blue  sky 
at  the  North  Pole  is  illuminated  by  the  solar  beams,  while  the  sun 
itself  may  be  still  under  the  horizon,  as  is  the  case  one  or  two  hours 
before  sunrise  or  as  much  after  sunset. 
Boric  Acid  in  Constipation. — Dr.  Platan,  of  Berlin,  suggests  the  insuffla- 
tion into  the  rectum  of  a  pinch  of  boric  acid,  to  relieve  constipation.  The  results 
are  said  to  be  excellent  even  in  severe  cases  in  which  mechanical  measures 
have  failed.  —Med.  News,  Feb.  7,  1891. 
