Kerr  Effects  in  the  Infra-red  Spectrum. 
59 
greatest  amount  of  energy  was  available  as  shown  in  the 
I  column  in  Table  I.,  are  the  more  accurate;  hence  the 
probable  error  increases  considerably  at  the  ends,  especially 
at  the  infra-red  end,  where  both  rotating-power  and  energy 
Fig.  2. 
Variation  of  the  Faraday  Effect  with  wave-length;  constant  field. 
2^  o' 
Wavelength 
4^- 
Present  work. 
Moreau's  results. 
are  rapidly  decreasing.  A  check  on  the  general  accuracy  of 
the  work  has  been  furnished  by  visual  measurements  with  a 
half-shade  polariscope  using  sodium  light.  These  gave  u 
rotation  of  170,(j  as  against  perhaps  17°*7  taken  from  the 
curve,  which  is  a  considerably  closer  agreement  than  might 
be  expected,  the  discrepancy,  indeed,  being  no  greater  than 
the  uncertainty  in  reading  a  point  from  the  curve. 
Table  II.  is  a  comparison  of  the  results  obtained  by  experi- 
ment, including  those  of  Moreau,  with  rotations  calculated  by 
means  of  the   four   principal    formulae   mentioned  above,  all 
