64 
Mr.  L.  R.  Ingersoll  on  the  Faraday  and 
and  about  3  rams,  thick.  Silver  was  included  in  the  tests  to 
indicate  if  any  external  cause  was  acting  to  produce  rotation, 
and  hence  give  rise  to  a  spurious  effect,  The  matter  of 
polish  >:<  was  rather  troublesome,  save  in  the  case  of  steel ;  so, 
to  avoid  as  much  as  possible  any  error  due  to  poor  surfaces, 
several  different  specimens  of  each  metal  were  generally 
tested,  and  those  re-polished  several  times. 
Results. 
The  results  are  shown  in  the  curves  of  figs.  3-6.  It  has 
not  been  thought  necessary  to  give  detailed  tables  of 
galvanometer-deflexions,  but  it  may  be  remarked  that,  by 
increasing  the  slit-width  and  brilliancv  of  the  glower,  these 
Fig.  3. — Variation  of  Kerr  Effect  with  wave-length.     Constant  held. 
Glass-plate  polarizing  apparatus. 
were  in  most  cases  made  large  enough  to  be  perfectly  definite, 
amounting  for  some  of  the  points  in  the  steel  curves  to  as 
much  as  30  cms.  for  values  of  2dl,  although  in  the  case 
of    nickel    they    were    rarely    more  than  4  or  5  cms.       It 
*  Just  whether  surface  imparities  have  any  appreciable  effect  on 
ihe  dispersion  in  the  Kerr  rotation  has  not  been  determined.  Micheli 
(Drude's  Ann.  i.  p.  565,  1900)  found  that  surface  impurities  tend  to  lessen 
the  "  critical  angle  "  as  observed  for  equatorial  magnetization,  but  no 
conclusions  can  be  drawn  from  that  to  apply  to  the  present  case. 
