584  PEOFESSOE  PLUCKEE  OX  THE  MAGNETIC  INDUCTION  OF  CETSTALS, 
another.  This  is  the  case  in  the  following  crystals : — Oxide  of  iron  (Eisenglanz),  yellow 
ferrocyanide  of  potassium,  cyanide  of  copper  and  potassium,  quartz,  sulphate  of  potash 
(whose  primitive  form  is  a rhombic  prism),  and  topaz. 
Note  added  during  the  'printing  of  the  paper. 
The  analogy  between  the  phenomena  presented  by  crystals  in  the  case  of  light  trans- 
mitted through  them,  and  in  the  case  of  magnetic  induction,  may  be  fully  explained  by 
the  circumstance  that  in  both  cases  the  laws  of  these  phenomena  depend  upon  an  auxiliary 
ellipsoid.  The  diameters  of  the  optic  auxiliary  ellipsoid  represent  the  values  of  the 
reciprocal  of  the  elasticity  of  the  ether  within  the  crystal ; two  of  its  axes  coincide  with 
the  directions  of  the  greatest  and  the  least  elasticity,  the  third  and  mean  axis  being 
perpendicular  to  these.  On  the  other  hand,  the  axes  of  the  'tnagnetic  auxiliary  ellipsoid 
are  directed  along  the  lines  of  the  greatest,  the  least,  and  the  mean  magnetic  induction, 
arrd  their  lengths  represent  the  values  of  the  reciprocal  of  these  three  inductions.  This 
analogy  becomes  the  more  perspicuous  and  striking  by  the  fact,  that  in  the  first  case 
the  optic  axes,  i.  e.  the  two  directions  along  which  there  is  no  double  refraction,  are  per- 
perrdicular  to  the  circular  sections  of  the  optic  auxiliary  ellipsoid ; while  in  the  second 
case,  the  magnetic  axes,  i.  e.  two  lines  such  that  the  crystal  on  being  suspended  along 
either  of  them,  betweerr  the  two  magnetic  poles,  is  not  acted  upon  by  these  poles  in  any 
extraordinary  way,  are  perpendicular  to  the  circular  sections  of  the  magnetic  auxiliary 
ellipsoid. 
In  crystals  with  only  one  principal  crystallographic  axis,  both  auxiliary  ellipsoids,  the 
optic  arrd  the  magnetic,  become  ellipsoids  of  revolutiorr  whose  principal  axis  coincides 
with  the  crystallographic  axis.  Iir  crystals  whose  primitive  form  is  a right  prism  with 
a rhombic  base,  the  three  axes  of  both  auxiliary  ellipsoids  are  directed  alorrg  the  three 
priircipal  crystallographic  axes,  but  in  both  cases  there  is  no  indication  at  all  given 
about  the  relative  length  of  the  three  axes.  In  crystals  belongirrg  to  the  monocluric 
system  there  is  only  orre  axis  common  to  both  auxiliary  ellipsoids,  tlris  axis  being 
perperrdicular  to  the  symmetric  plane ; the  two  remainiirg  axes  of  both  ellipsoids  lie  in 
this  plane,  where  their  position,  not  indicated  by  airy  general  law,  nray  be  easily  found 
by  observation.  In  triclinic  crystals  there  is  no  indication  whatever  given  by  the 
crystallographic  form,  regarding  the  positioir  of  the  axes  of  the  two  ellipsoids,  and  there- 
fore the  determination  of  these  axes  is  more  difficult. 
When  a plane  luminous  wave  is  transmitted  through  a crystal  along  any  direction, 
the  vibrations  in  the  front  of  the  wave  take  place  along  the  two  axes  of  the  ellipse,  in 
which  the  optic  auxiliary  ellipsoid  is  intersected  by  the  front.  Experimentally  these 
directions  are  determined  by  puttmg  a plate  of  the  crystal,  bounded  by  faces  parallel  to 
the  troirt  of  the  wave,  oir  a polarizing  apparatus,  arrd  by  turning  it  till  it  appears  dark. 
Then  one  of  the  two  directions  of  vibratioir  lies  in  the  primitive  plarre  of  polarization. 
