Kerr  Effects  in  the  Infra-red  Spectrum. 
61 
The  constants  a  and  b,  a1  and  b\  are  those  given  by  Drude 
for  the  formulae  as  applied  to  rotatory  dispersion  in  the  visible 
spectrum,  c  and  cf  have  been  determined  for  wave-length  ?>fjL. 
Their  presence  has  practically  no  effect  on  the  formulae  as 
applied  to  the  visible  spectrum,  so  the  validity  in  that  region 
has  not  been  sacrificed.  The  values  of  n  beyond  X=2  //,  used 
in  calculations  with  these  two  formulae  have  been  obtained  by 
prolonging  the  dispersion-curve  already  known.  Unless  the 
ordinary  dispersion  of  carbon  bisulphide  between  2  //,  and 
4  fju  is  very  anomalous,  which  is  unlikely,  the  values  so 
obtained  are  more  than  sufficiently  accurate  for  the  purpose. 
As  may  be  seen,  both  the  formulae  of  Drude,  as  modified, 
fit  the  case  much  better  than  the  others.  As  to  distinguishing 
between  them,  throughout  the  greater  range  of  spectrum 
tested  neither  has  a  decided  advantage,  although  the  first  may 
show  a  slightly  better  agreement.  But  beyond  A.r=3'50//,, 
where  the  curve  is  very  flat,  and  indeed  appears  to  take  a 
slight  upward  turn,  the  state  of  affairs  is  much  better  repre- 
sented by  the  second  formula,  which  is  the  one  deduced  on 
the  supposition  of  the  Hall  effect.  The  points  calculated 
by  this  formula  have  been  plotted  in  fig.  2,  each  beino* 
denoted  by  the  apex  of  an  inverted  letter  V. 
Table  II. 
Wave- 
length. 
Ob- 
served. 
k. 
•589 
1-00 
•700 
•660 
•945 
•332 
1  070 
•272 
1-120 
•248 
1-170 
•231 
1-419 
■\r^ 
1-80 
•099 
2  00 
•079 
2-50 
■052 
300 
•041 
3-50 
'036 
4-GO 
•037 
\  •3.0 
•012 
M( 
11)0 
•48 
•44 
Forn 
ulte  of 
Joubin. 
Maxwell. 
1    Drude 
Drude 
1-00 
I  00 
1-00 
1-00 
•675 
•670 
•44 
•35 
■335 
•3-1 S 
•38 
•28 
•271 
■265 
•35 
•25 
•245 
•242 
•32 
••)•) 
■22<j 
■28 
•13 
•157 
•152 
•09S 
095 
•oo 
•osi 
078 
•055 
■ox\ 
•on 
•on 
•032 
•037 
( I2S 
•088 
026 
on 
