[    310    ]  • 
XXXVIII.  Proceedings  of  Learned  Societies. 
ROYAL  SOCIETY. 
f Continued  from  p.  234.] 
Nov.  23,  1871.— General  Sir  Edward  Sabine,  K.C.B.,  President, 
followed  by  Mr.  Francis  Galton,  Vice-President,  in  the  Chair. 
rpHE  following  communication  was  read  ■ — 
-■-  "On  a  supposed  Alteration  in  the  amount  of  Astronomical 
Aberration  of  Light,  produced  by  the  passage  of  Light  through  a 
considerable  thickness  of  Refracting  Medium."  By  George  Biddell 
Airy,  C.B.,  Astronomer  Royal. 
A  discussion  has  taken  place  on  the  Continent,  conducted  partly 
in  the  '  Astronomische  Nachrichten,'  partly  in  independent  pam- 
phlets, on  the  change  of  direction  which  a  ray  of  light  will  receive 
(as  inferred  from  the  Undulatory  Theory  of  Light)  when  it  traverses 
a  refracting  medium  which  has  amotion  of  translation.  The  subject 
to  which  attention  is  particularly  called  is  the  effect  that  will  be  pro- 
duced on  the  apparent  amount  of  that  angular  displacement  of  a 
star  or  planet  which  is  caused  by  the  Earth's  motion  of  translation, 
and  is  known  as  the  Aberration  of  Light.  It  has  been  conceived  that 
there  may  be  a  difference  in  the  amounts  of  this  displacement  as 
seen  with  different  telescopes,  depending  on  the  difference  in  the 
thicknesses  of  their  object-glasses.  The  most  important  of  the  papers 
containing  this  discussion  are : — that  of  Professor  Klinkerfues,  con- 
tained in  a  pamphlet  published  at  Leipzig  in  1867,  August;  and 
those  of  M.  Hoek,  one  published  1867,  October,  in  No.  1669  of  the 
'  Astronomische  Nachrichten/  and  the  other  published  in  1869  in  a 
communication  to  the  Netherlands  Royal  Academy  of  Sciences. 
Professor  Klinkerfues  maintained  that,  as  a  necessary  result  of  the 
Undulatory  Theory,  the  amount  of  Aberration  would  be  increased, 
in  accordance  with  a  formula  which  he  has  given  ;  and  he  sup- 
ported it  by  the  following  experiment : — 
In  the  telescope  of  a  transit-instrument,  whose  focal  length  was 
about  18  inches,  was  inserted  a  column  of  water  8  inches  in  length, 
carried  in  a  tube  whose  ends  were  closed  with  glass  plates ;  and 
with  this  instrument  he  observed  the  transit  of  the  Sun,  and  the 
transits  of  certain  stars  whose  north-polar  distances  were  nearly  the 
same  as  that  of  the  Sun,  and  which  passed  the  meridian  nearly  at 
midnight.  In  these  relative  positions,  the  difference  between  the 
Apparent  Right  Ascension  of  the  Sun  and  those  of  the  stars  is  affected 
by  double  the  coefficient  of  Aberration  ;  and  the  merely  astrono- 
mical circumstances  are  extremely  favourable  for  the  accurate  test- 
ing of  the  theory.  Professor  Klinkerfues  had  computed  that  the 
effect  of  the  8-inch  column  of  wrater  and  of  a  prism  in  the  interior 
of  the  telescope  would  be  to  increase  the  coefficient  of  Aberration 
by  eight  seconds  of  arc.     The  observation  appeared  to  show  that 
