322  The  Hon.  J.  W.  Strutt  on  the  Reflection  and 
than  a  few  wave-lengths  within  the  medium.  There  can  be 
little  doubt  that  in  this  Cauchy  was  perfectly  right ;  for  it  has 
been  found  that  bodies  which,  like  many  of  the  dyes,  exercise  a 
very  intense  selective  absorption  on  light,  reflect  from  their  sur- 
faces in  excessive  proportion  just  those  rays  to  which  they  are 
most  opaque.  Permanganate  of  potash  is  a  beautiful  example 
of  this,  given  by  Professor  Stokes.  He  found  (Phil.  Mag. 
vol.  vi.  p.  293)  that  when  the  light  reflected  from  a  crystal  at 
the  polarizing  angle  is  examined  through  a  Nicol  held  so  as  to 
extinguish  the  rays  polarized  in  the  plane  of  incidence,  the  resi- 
dual light  is  green,  and  that,  when  analyzed  by  the  prism,  it 
shows  bright  bands  just  where  the  absorption-spectrum  shows 
dark  ones.  This  very  instructive  experiment  can  be  repeated 
with  ease  by  using  sunlight,  and  instead  of  a  crystal  a  piece  of 
ground  glass  sprinkled  with  a  little  of  the  powdered  salt,  which 
is  then  well  rubbed  in  and  burnished  with  a  glass  stopper  or 
otherwise.  It  can  without  difficulty  be  so  arranged  that  the 
two  spectra  are  seen  from  the  same  slit  one  over  the  other,  and 
compared  with  accuracy. 
With  regard  to  the  chromatic  variations,  it  would  have  seemed 
most  natural  to  suppose  that  the  opacity  may  vary  in  an  arbi- 
trary manner  with  the  wave-length,  while  the  optical  density  (on 
which  alone  in  ordinary  cases  the  refraction  depends)  remains 
constant,  or  is  subject  only  to  the  same  sort  of  variations  as 
occur  in  transparent  media.  But  the  aspect  of  the  question  has 
been  materially  changed  by  the  observations  of  Christiansen  and 
Kundt  (Pogg.  Ann.  vols,  cxli.,  cxliii.,  cxliv.)  on  anomalous  dis- 
persion in  Fuchsin  and  other  colouring-matters,  which  show 
that  on  either  side  of  an  absorption-band  there  is  an  abnormal 
change  in  the  refrangibility  (as  determined  by  prismatic  devia- 
tion) of  such  a  kind  that  the  refraction  is  increased  below  (that 
is,  on  the  red  side  of)  the  band  and  diminished  above  it.  An 
analogy  may  be  traced  here  with  the  repulsion  between  two  pe- 
riods which  frequently  occurs  in  vibrating  systems.  The  effect 
of  a  pendulum  suspended  from  a  body  subject  to  horizontal  vi- 
bration is  to  increase  or  diminish  the  virtual  inertia  of  the  mass 
according  as  the  natural  period  of  the  pendulum  is  shorter  or 
longer  than  that  of  its  point  of  suspension.  This  may  be  ex- 
pressed by  saying  that  if  the  point  of  support  tends  to  vibrate 
more  rapidly  than  the  pendulum,  it  is  made  to  go  faster  still, 
and  vice  versa.  Below  the  absorption-band  the  material  vibra- 
tion is  naturally  the  higher,  and  hence  the  effect  of  the  associated 
matter  is  to  increase  (abnormally)  the  virtual  inertia  of  the  aether, 
and  therefore  the  refrangibility.  On  the  other  side  the  effect  is 
the  reverse*.  It  would  be  difficult  to  exaggerate  the  import- 
*  See  Sellmeier,  Pogg.  Ann.  vol.  cxliii.  p.  272. 
