£6 
Mr.  L.  R.  Ingersoll  on  the  Faraday  and 
and  nicol.  They  allowed  measurement  as  far  as  X=2*4//,  in 
the  infra-red,  and  this  was  almost  as  far  as  dependable  results 
could  be  obtained  with  the  use  of  the  plates.  While  certain 
characteristic  differences  appear  between  the  curves  obtained 
with  the  two  different  polarizing  arrangements, — both  in  the 
Fig.  5. — Variation  of  Kerr  Effect  with  wave-length. 
Glass-plate  polarizing  apparatus. 
-80 
Constant  field. 
2^ 
WAV£ LENGTH 
matter  of  form,  which  may  be  due  to  errors  arising  from 
oblique  incidence  at  the  surface  of  the  nicol,  and  as  regards 
absolute  magnitude,  which  appears  somewhat  greater  when 
measured  with  the  pile  of  plates, — the  results  are  on  the 
whole  consistent.  The  curves  of  fig.  4  and  6  were  made 
with  the  doubly  refracting  arrangement. 
The  first  striking  feature  which  appeared  in  the  results  was 
the  marked  diminution  of  rotation  with  increase  of  wave- 
length, as  is  shown  in  the  curves,  instead  of  the  reverse  effect 
which  was  to  be  expected  from  the  results  of  observations  on 
the  visible  part  of  the  spectrum.  This  indicated  the  existence 
of  maxima  which,  in  the  cases  of  the  three  magnetic  metals, 
appeared  to  lie  somewhat  to  the  left  of  1  //,,  but  did  not  prove 
easy  to  obtain  because  dependent  on  points  just  beyond  the 
range  of  the  visible  and  not  far  enough  into  the  infra-red  for 
accurate  measurement,  because  of  lack  of  energy.  However, 
in  several  cases  the    maxima   were   clearly  indicated,  while 
