60  Mr.  L.  R.  Ingersoll  on  tlte  Faraday  and 
results,  for  the  sake  of  comparison,  being  brought  to  the  same 
scale  by  letting  the  rotation  for  the  D  line  equal  unity.  The 
figures  for  the  formula)  of  Joubin  and  Maxwell  are  taken  from 
a  table  given  by  M oreau  *.  The  formula  is,  as  given  by  this 
writer, 
S  =  C.r+C  -,  where  m  =  — A-— . 
A,  A,  CiA, 
Log  0  =  1*29814  and  log  C  =  2*43254,  if  X  is  expressed  in 
terms  of  hundredth  microns. 
In  the  formula  of  Maxwell, 
S{«-\SW4}> 
log  £=2*87687,  and  log  &'=4*11114.  The  figures  given  by 
Moreau  have  been  extended  to  \  =  2'0  /u,  which,  if  the  writer's 
knowledge  is  correct,  is  as  far  as  the  refractive  index  of 
carbon  bisulphide  is  accurately  known.  The  formula  of 
van  Schaik  gives  almost  identical  results  with  this  of  Maxwell 
in  the  infra-red,  and  hence  need  not  be  further  mentioned. 
The  formulae  of  Drude  without  modification  are  inadequate, 
for  the  presence  of  the  carbon-bisulphide  absorption-band  at 
X='212  fji  will  be  of  little  effect  in  the  infra-red,  and  will  be 
entirely  insufficient  to  cause  the  singular  flatness  of  the  curve 
in  the  neighbourhood  of  3*5  /x.  However,  carbon  bisulphide 
has  also  a  well-marked  absorption-band  at  8*05  yu-t,  and  by 
adding  a  term  which  shall  take  account  of  this  in  the  same 
way  that  the  second  term  does  the  ultra-violet  band,  it  is 
possible  to  fit  the  formulae  more  closely  to  the  facts. 
Accordingly  they  are  written 
*         (  a  b  c       "") 
where 
a= -0*0136  :   &=+0*1530T-e="^0*2370; 
V  =  "045  ;    A22  =  64*8; 
and  „_  1  (  a^  l/\^_  c'X2        \ 
where 
n  (  A2   '    (X2-V)2       (^2- V)" 
a'=  +0-1167;    &'= +0*2379;    c'= +3*482. 
*  Zoc.  cit. 
t  Angstrom  places  this  band  at  X=8"4  /x  (Kayser's  •  Spectroscopies 
Bd.  iii.p.  371),  which  value  would  serve  the  purpose  almost  as  well  as  the 
above  with  a  sliffhtlv  different  choice  of  constants. 
