PEOFESSOE  PLiiCEIEE  ON  THE  MAGNETIC  INDUCTION  OP  CEYSTALS.  587 
By  turning,  within  their  plane,  each  of  the  two  optic  axes  for  violet  light  in  the  same 
direction,  these  axes  will  pass  into  the  position  of  the  axes  for  red  light,  and  afterwards, 
by  turning  still  in  the  same  direction,  into  the  position  of  the  magnetic  axes.  In  com- 
mon with  Professor  Beer,  I examined  a great  number  of  crystals,  in  order  to  find  a 
general  law  between  the  position  of  the  magnetic  axes  of  a crystal  and  its  optic  axes 
for  the  different  colours,  but  without  a satisfactory  result.  The  laws  mentioned  in  the 
second  paragraph  of  these  additional  pages,  by  means  of  which  we  can  in  the  simpler 
cases  deduce  from  the  primitive  form  of  a crystal  the  position  of  the  axes  of  both  auxi- 
liary ellipsoids,  the  magnetic  and  the  optic — and  also  of  all  such  auxiliary  ellipsoids 
regarding,  for  instance,  molecular  elasticity,  conduction  of  heat  and  electricity — are  all 
we  know. 
I added  this  note  to  my  original  paper,  in  order  to  explain  more  distinctly  than  I did 
before  the  analogy  between  the  optic  and  magnetic  axes,  which  guided  me  during  the 
different  stages  of  my  researches  on  the  magnetic  induction  of  crystals.  My  intention 
was  not  at  all  to  enter  into  the  mathematical  solution  of  the  great  physical  problem. 
All  my  experimental  researches  follow  from  the  mere  fact,  that  there  exists  an  auxiliary 
ellipsoid  of  magnetic  induction.  The  supposition  of  molecular  ellipsoids  of  isotropic, 
paramagnetic  or  diamagnetic  matter,  is  to  be  regarded  as  a means  of  discovering  the 
mathematical  laws  to  which  the  couple,  tending  to  turn  round  a magnecrystal  in  a uni- 
form field,  was  subject,  and  not  as  establishing  these  laws  on  a basis  of  molecular  physics. 
Such  a basis  is  to  be  obtained  by  Professor  W.  Thomson’s  theory  only,  which  I highly 
regret  not  to  have  known  when  I wrote  my  paper.  From  this  theory  too,  an  abstract 
of  which  appeared  in  the  Philosophical  Magazine  (March  1851),  the  above-mentioned 
auxiliary  ellipsoid  follows.  Theoretically  speaking,  it  was  wrong  to  replace  it  by  Pois- 
son’s auxiliary  ellipsoid,  but  this  mistake  does  not  in  the  least  way  affect  the  object  of 
the  paper  presented  to  the  Royal  Society. 
