Prof.  Challis  on  the  Theory  of  the  Aberration  of  Light.     239 
maximum  ;  while  a  distinct  displacement  occurred  with  a  layer 
of  10  millims.  thickness. 
Permit  me  to  add  one  remark,  that,  from  all  the  experiments 
I  have  made  on  the  ultra-red  rays  of  the  solar  spectrum  in  differ- 
ent hours  of  the  forenoon,  I  hold  myself  justified  in  the  assump- 
tion that  the  absorptive  part  of  the  atmosphere  follows  the  same 
law;  for  I  have  always  seen  that  the  intensity  of  the  rays  of  less 
refrangibility  was  always  less  in  the  morning,  when  the  solar 
rays  had  to  travel  a  much  longer  path  through  our  humid  atmo- 
sphere, than  towards  noon. 
XXXIV.   On  the  Theory  of  the  Aberration  of  Light. 
By  Professor  Challis,  M.A.,  LL.D.,  F.R.S.* 
SINCE  the  publication  of  my  "  Note  on  the  Aberration  of 
Light "  in  the  Number  of  the  Philosophical  Magazine  for 
June  1855,  several  experiments  have  been  made  for  the  purpose 
of  settling  a  question  which  was  incidentally  adverted  to  in  that 
communication — namely,  whether  or  not  any  effect  is  produced 
on  the  amount  of  aberration  by  the  transmission  of  the  light 
through  the  lenses  of  the  telescope.  The  experiments  I  refer  to 
are : — those  of  M.  Klinkerfues,  described  in  a  pamphlet  entitled 
Die  Aberration  der  Fixsterne  nach  der  Wellentheorie,  which  was 
published  at  Leipsic  in  1867;  those  of  M.  Hoek,  the  details  of 
which  are  given  in  No.  1741  of  the  Astronomische  Nachrichten ; 
and  the  experiments  made  last  year  at  the  Greenwich  Observa- 
tory, the  results  of  which  are  stated  by  the  Astronomer  Royal  in 
No.  130  of  the  f  Proceedings  of  the  Royal  Society '  (p.  35), 
where  also  reference  is  made  to  the  discussion  of  the  question 
by  the  two  experimenters  just  named.  Each  of  these  sets  of 
experiments  was  made  with  a  telescope  the  tube  of  which  was 
filled  through  a  certain  length  with  a  fluid.  M.  Klinkerfues, 
who  used  oil  of  turpentine,  pointed  his  telescope  to  the  sun  and 
to  stars,  and  supposed  that  his  observations  gave  considerable 
aberration  due  to  the  fluid.  M.  Hoek  contested  this  inference  in 
Nos.l669and  17-11  of  the  Astronomische  Nachrichten,  and  showed 
that  with  a  telescope  partly  filled  with  water  and  directed  to  a 
terrestrial  object  no  aberration  could  be  detected.  This  result, 
however,  did  not  conclusively  prove  that  no  aberration  due  to 
the  water  would  have  been  obtained  if  the  telescope  had  been 
directed  to  a  star.  The  Greenwich  observations  were  made 
entirely  on  7  Draconis,  and  gave,  on  being  reduced,  an  amount 
of  aberration  not  sensibly  differing  from  that  in  the  '  Nautical 
Almanac/  and  thus  showed  that  very  little  or  no  aberration  was 
*  Communicated  by  the  Author. 
Phil  Mag.  S.  4.  Vol.  43.  No.  286.  April  1872.  U 
