Am.  Jour.  Pharm 
Jan.,  1880 
Polar imeter  and  its  Use. 
45 
Laurent  practically  in  his  business  for  determining  the  purity  of  certain  alkaloids,, 
etc.,  and  was  assured  by  him  that  the  results  obtained  were  as  trustworthy  as  those  of 
the  most  accurate  chemical  analysis.  To  accomplish  the  object  already  mentioned 
and  render  the  subject  thoroughly  clear  to  those  who  have  not  previously  given  any 
attention  to  it,  I  may  be  allowed  to  say  a  few  words  on  polarized  light. 
A  ray  of  common  light,  as  you  will  be  aware,  is  assumed  to  con'<ist  of  vibrations 
in  the  ethereal  medium  or  luminiferous  ether,  occurring  in  two  directions  at  right 
angles  to  each  other,  ?nd  by  interference  the  primary  planes  are  constantly  shifting. 
If,  however,  these  two  vibrations  are  split  up  by  the  absorption,  reflection  or  dis- 
persion of  one,  or  by  refraction  of  both,  the  remaining  portion,  or  one  of  the  portions 
separated,  constitutes  a  ray  of  polarized  light,  and  as  the  phenomenon  of  interference 
ceases  il  vibrates  in  one  plane  only.  If,  now,  this  is  made  to  traverse  certain  media,, 
the  plane  no  longer  remains  in  this  direction,  but  is  deviated  either  to  the  right  or 
left,  and  is  caused  to  rotate  or  assume  a  spiral  form,  and  it  is,  as  already  stated,  for 
the  measurement  of  the  amount  of  rotation  caused  by  different  fluids  when  so  trav- 
ersed that  the  polarimeter  has  been  constructed. 
The  property  possessed  by  quartz  of  circularly  polarizing  a  ray  of  light  was  known 
to  Sebeck  and  Arago,  but  it  is  to  Biot,  in  1818,  that  we  owe  the  discovery  of  the 
property  possessed  by  many  fluids  of  rotating  a  ray  of  plane  polarized  light.  He 
states  that  this  occurred  to  him  accidentally  whilst  examining  crystallized  laminas,. 
placed  in  highly  refractive  media,  such  as  oil  of  turpentine.  He  thoroughly  investi- 
gated the  phenomenon,  and  laid  the  foundation  of  a  very  important  study,  his  early 
results  being  obtained  by  means  of  an  instrument  devised  by  himself,  not  unlike  the 
polariscope  attached  to  the  microscope,  except  that  the  polarization  was  obtained  by 
reflection  from  a  blackened  mirror,  and  that  the  analyzer  was  placed  in  the  centre  of 
a  graduated  disc.  When  the  analyzing  prism  was  so  placed  as  to  obscure  the 
polarized  ray,  on  interposing  a  tube  containing  an  active  fluid  the  light  was  again 
found  to  pass  until  the  analyzer  had  been  rotated  through  a  certain  number  of 
degrees,  that  number  being  taken  as  the  rotatory  power  of  the  fluid;  but  it  wa& 
found  difRcult  to  determine  the  exact  point  of  maximum  darkness,  and  somewhat 
wide  and  inaccurate  results  were  obtained.  M.  Soieil,  an  instrument  maker  of  Paris, 
next  constructed  with  considerable  ingenuity  and  skill  an  improved  form,  by  the  use 
of  which  much  greater  accuracy  could  be  obtained.  In  it  the  light  first  passes- 
through  a  double  refracting  prism  as  analyzer,  then  through  a  plate  of  quartz  3.75, 
mm.  thick  (subsequently  replaced  by  a  double  plate),  then  through  the  fluid  under 
examination,  another  plate  of  quartz,  the  compensator  consisting  of  two  wedges  of 
quartz,  and  finally  through  the  analyzer.  To  this  there  was  added  what  Soleil  called 
a  produce  of  sensible  tintSy  consisting  of  a  prism,  Galileo  telescope  and  quartz  plate. 
On  one  occasion  I  spent  a  profitable  hour  or  two  in  thoroughly  examining  this 
instrument,  taking  it  to  pieces  and  tracing  the  tortuous  course  of  a  ray  of  light 
through  it.  The  study  was  interesting  as  showing  what  optical  skill  can  accomplish 
and  what  complicated  means  had  here  been  employed  to  surmount  difficulties,  which, 
have  since  been  overcome  in  a  more  simple  manner.  The  special  features  in  this 
instrument  arc  first,  that  the  ray  of  polarized  light  emerging  in  a  vertical  plane  froni. 
the  prism  meets  the  double  plate  of  quartz,  one-half  of  which  rotates  to  the  rights 
