MOLECULAR  DISSYMMETRY  OF    ORGANIC  PRODUCTS.  3 
bodies  ;  in  other  words,  that  the  two  rays,  ordinary  and  extra- 
ordinary, given  by  a  bi-refrangent  crystal,  are  polarized  rays. 
Mains  established  so  clearly  from  the  beginning,  these  fruitful 
discoveries,  with  so  much  care  and  so  much  precision  in  his 
facts  and  in  his  language,  that  one  might  believe,  in  reading  his 
memoirs,  that  they  were  prepared  yesterday.  But  he  could 
not  pursue  his  work :  premature  death  carried  him  off  in  1812, 
at  the  age  of  thirty-seven  years.  Happily  for  science,  two 
celebrated  physicists,  at  that  time  young  and  full  of  activity, 
MM.  Biot  and  Arago,  received  his  legacy,  and  were  not  slow 
to  distinguish  themselves  by  brilliant  discoveries  in  the  new 
route  which  Malus  had  just  opened  to  science. 
In  1811.,  Arago  observed,  that  when  a  polarized  ray  traversed 
normally  a  plate  of  rock  crystal  cut  perpendicularly  to  its  axis, 
the  ray,  on  issuing  from  the  plate,  if  analyzed  by  aid  of  a 
rhomboid  of  Iceland  spar,  gives  constantly  tivo  images  in  every 
position  of  the  rhomboid ;  and  further,  these  two  images  are 
colored  with  the  complementary  tints.  When  the  thickness  of 
the  spar  does  not  permit  an  entire  separation  of  the  two 
fasciculi,  the  image  is  white  where  they  are  in  part  superposed. 
This  experiment  presents  a  double  anomaly  to  the  ordinary 
laws  of  bi-refrangent  crystals.  Every  other  crystal,  with  an 
axis  cut  normally  to  this  axis,  would  have  furnished  two  white 
images,  in  place  of  being  colored,  and,  in  two  rectangular  posi- 
tions of  the  rhomboid  analyzer,  the  images  would  be  reduced  to 
a  single  one. 
The  conclusion  of  Arago  was,  that  the  results  of  the  preced- 
ing experiment  are  precisely  those  which  should  follow,  if  we 
suppose  that  the  differently  colored  rays  of  the  incident  white 
fasciculus  are,  on  issuing  from  the  plate  of  the  quarts,  polarized 
indifferent  planes. 
Arago  does  not  recur  to  these  brilliant  phenomena,  the  laws 
of  which  M.  Biot  presented  since  1813,  carefully  isolating 
them  from  those  among  which  Arago  appeared  to  confound 
them. 
M.  Biot  formed  the  polarized  ray  successively  with  each  of 
the  simple  rays  of  the  spectrum,  and  found  that  the  plane  of 
primitive  polarization  was  deviated  at  an  angle  proportional  to 
the  thickness  of  the  plate  ;  that  this  angle  is  different  for  each 
