296       M.  O.  E.  Meyer  ■  on  Anomalous  Dispersion  of  Light. 
anomalous  dispersion  in  a  very  great  number  of  substances  has 
turned  general  attention  to  the  subject. 
Anomalous  dispersion  of  light  appears  to  occur  only  in  those 
bodies  which  very  strongly  absorb  light,  so  as  to  be  almost 
opaque.  This  coincidence  is  the  more  striking,  as  at  the  same 
time  other  optical  properties  are  almost  always  found  in  combi- 
nation with  these  two  :  namely,  the  bodies  exhibit  likewise  elliptic 
polarization  of  the  reflected  light,  and  frequently  a  surface-colour 
different  from  the  body-colour. 
That  elliptic  polarization  and  anomalous  dispersion  are  com- 
bined in  the  case  of  metals  was  observed  by  Brewster*.  In 
Cauchy's  theory  of  metallic  reflection^  elliptic  polarization  ap- 
pears to  be  necessarily  united  with  strong  absorption,  i.  e.  opa- 
city ;  this  we  learn  from  the  structure  of  his  formulae,  in  which, 
to  the  periodic  functions  representing  the  oscillations,  exponen- 
tial functions  are  added  which  measure  the  weakening  of  the 
light.  It  is  moreover  well  known  that  several  metals,  and  gold 
above  all,  transmit  light  of  a  different  colour  from  that  which 
they  reflect — that  is,  possess  a  body-colour  different  from  the 
colour  of  their  surface. 
The  same  number  of  properties  are  found  united  in  other 
classes  of  bodies  besides  the  metals — namely,  in  the  many 
strongly  coloured  substances  whose  anomalous  dispersion  has 
been  observed  by  LerouxJ,  Christiansen §,  Kundt  ||,  and  Soret^f. 
Among  these  bodies,  Carthamin  must  above  all  be  mentioned 
as  that  substance  of  which  all  those  properties  have  been  clearly 
demonstrated.  Stokes  in  1853  **  referred  to  the  close  connexion, 
in  these  bodies,  between  elliptic  polarization  of  reflected  light, 
their  different  body-  and  surface-colours,  and  their  strong  ab- 
sorptive power.  Since  Kundt  has  now  recently  indicated,  in  his 
first  memoir,  the  connexion  of  anomalous  dispersion  with  the 
surface-colour,  and,  in  his  second  and  third,  with  the  absorption 
of  light,  it  can  hardly  be  doubted  that  all  these  properties  occur 
simultaneously  in  non-metallic  bodies  also. 
It  is  natural  to  regard  as  causal  the  most  striking  of  these 
properties,  that  which  shows  itself  most  distinctly  with  all — 
that  is,  opacity.  This  will  be  most  simply  explained  by  the  hy- 
pothesis of  a  resistance  undergone  by  the  oscillating  particles  of 
*  Phil.  Trans.  1830,  p.  325.     Pogg.  Ann.  vol.  xxi.  (1831). 
f  Comptes  Rendus,  vol.  viii.  p.  553  (1839).  Compare  Beer,  Pogg.  Ann. 
vol.  xci.  p.  561  (1854);  and  F.  Eisenlohr,  Pogg.  Ann.  vol.  civ.  p.  368, 
(1858). 
X  Comptes  Rendus,  vol.  lv.  p.  126.     Phil.  Mag.  S.  4.  vol.  xxiv.  p.  245. 
§  Pogg.  Ann.  vol.  cxliii.  p.  250(18/1).     Phil.  Mag.  S.  4.  vol.  xli.  p.  244. 
||   Pogg.  Ann.  vol.  cxl.  p.  163;  vol.cxliii.  p.  259;  vol.  cxliv.  p.  128(1871). 
IT  Arch,  des  Sc.  Phys.  March  18/1.     Pogg.  Ann.vol.  cxliii.  p.  325. 
**  Phil.  Mag.  S.  vol.  vi.  p.  293.     Pogg.  Ann.  vol.  xci.  p.  300. 
