44  Mr.  L.  R.  Ingersoll  on  the  Faraday  and 
Dispersion  Formulae. 
It  is  not  the  intention  to  enter  into  a  discussion  of  the 
various  theories  o£  magneto-optic  phenomena  which  have 
been  proposed  by  Maxwell,  Lorentz,  Groldhammer,  Drude, 
and  others,  but  rather  to  test,  over  a  much  wider  range  of 
spectrum  than  has  heretofore  been  possible,  the  various 
rotatory-dispersion  formulae  deduced  on  the  basis  of  each, 
and  thus  perhaps  to  furnish  more  evidence  in  favour  of  some 
one  of  them,  or  a  modification. 
Among  such  formulae  may  be  mentioned  that  due  to 
Maxwell  and  formulated  by  Moreau  *, 
~      n2  (         ^  dn^  ( 7      _ ,  n2  \ 
and  the  very  similar  one  of  Van  Schaik  f, 
also  that  due  to  Joubin  J, 
=H-C'; 
< 
To  these  must  be  added  the  formulae  given  by  Drude  §, 
{ 
«+^^ 
X2^\2-\l2j  ' 
deduced  on  the  hypothesis  of  molecular  currents  ;  and 
n   l\s  +  (\a-V)sJ' 
deduced  on  the  hypothesis  of  the  Hall  effect,  \x  being  in  each 
case  the  wave-length  of  an  ultra-violet  absorption-band. 
With  the  data  hitherto  obtained,  it  is  impossible  to  decide 
definitely  in  favour  of  any  one  of  these,  for  over  the  limited 
range  of  spectrum  over  which  they  have  been  satisfactorily 
tested — barely  half  a  micron — it  is  possible,  by  properly 
choosing  the  constants,,  to  make  any  one  of  them  fit  the  facts 
sufficiently  well.  Moreau's  work  in  the  infra-red  points  in  a 
general  way  to  the  formula  of  Joubin,  although  this  is  hardly 
as  satisfactory  as  the  others  in  the  visible;  but,  as  will  be  seen 
later,  the  points  he  obtained  do  not  lie  at  all  on  a  regular 
curve,  and  it  is  possible  that  his  experimental  error  is  larger 
than  estimated. 
*  Ann.  Chim.  Pkys.  [7]  i.  p.  341  (1894). 
t  Arch.  Neer.  xvii.  (1882)  and  xxi.  (1880). 
%  Ann.  Chim.  Phys.  [6n  xvi.  p.  78  (1880). 
$  Lehrbuch  dor  Optt'k.  pp.  306  &  403. 
