382  Royal  Society : — 
In  the  spectroscope  the  three  bands  are  distinctly  seen.  The 
position  in  the  spectrum  of  the  least  refrangible  band  corresponds 
with  the  first  band  of  the  carbon  spectrum ;  it  commences  from  the 
red,  with  a  wave-length  of  about  5632mm. 
Attempts  were  made  with  a  double-image  prism,  a  Nicol's  prism, 
and  a  Nicol's  prism  combined  with  a  Savart's  system  of  plates  to  de- 
tect polarized  light  in  the  comet,  but  without  success. 
Nov.  1 4.  The  form  of  the  comet  remains  nearly  the  same.  The 
outlying  nebulosity  is  now  chiefly  on  the  south  of  the  axis  of  the 
comet.  The  nucleus  appears  to  be  precisely  at  the  extreme  eastern 
limit  of  the  brighter,  more  condensed  part  of  the  comet. 
The  same  spectrum  was  seen,  but  fog  coming  on  interrupted  the 
observations. 
On  this  evening  an  attempt  was  made  again  to  detect  polarized 
light.  A  double-image  prism  was  placed  between  the  eyepiece  and 
the  eye.  The  prism  was  brought  into  four  different  positions  90° 
apart.  At  each  position  of  the  prism  an  attempt  was  made  to  esti- 
mate the  relative  brightness  of  the  two  images.  The  power  of  the 
prism  was  just  sufficient  to  give  two  images  of  the  comet  without 
their  overlapping.  The  difference  in  brightness  of  the  images  was 
exceedingly  small ;  I  could  not  be  certain  that  any  appreciable  differ- 
ence really  existed.  However,  I  attempted  in  each  case  to  select  one 
of  the  two  images  as  the  brighter  one.  Afterwards  I  determined 
the  position  of  the  prism  at  the  four  different  estimations,  and  I 
then  found  that  three  of  the  estimations  were  in  accordance  with  a 
portion  of  the  comet's  light  being  polarized  in  a  plane  passing 
through  the  sun,  and  one  in  opposition  to  that  supposition.  I  he- 
sitate to  attach  any  positive  value  to  these  observations ;  but  they 
may  perhaps  be  taken  as  showing  that  no  considerable  part  of  the 
comet's  light  is  polarized. 
The  foregoing  observations  appear  to  show  that  the  spectrum  of 
this  comet  is  identical  with  that  of  Comet  II.  1868,  a  description  of 
which  I  had  the  honour  to  present  to  the  Royal  Society*. 
It  is  worthy  of  notice  that  the  cometary  matter  appears  drawn  out 
and  diffused  towards  the  sun,  and  that  it  has  not  yet  come  under  the 
influence  of  the  force,  or  been  subjected  to  the  conditions,  whatever 
they  may  be,  by  which  in  most  cases  cometary  matter  appears  to  be 
powerfully  repelled  from  the  sun. 
The  observations  were  made  with  the  telescope  belonging  to  the 
Royal  Society,  of  1 5  inches  aperture.  The  spectroscope  contained 
one  prism  with  a  refracting  angle  of  60,  and  the  small  observing 
telescope  magnified  six  times.  & 
Dec.  7. — George  Biddell  Airy,  C.B.,  President,  in  the  Chair. 
The  following  communication  was  read  : — 
"  On  Fluoride  of  Silver.— Part  111."     By  G.  Gore,  F.R.S. 
In  this  communication  the  author  has  finally  shown  that  the  action 
of  iodine,  under  the  influence  of  heat  (including  the  process  described 
by  Kammerer,  Phil.  Mag.  1863,  vol.  xxv.  p.  213,  for  the  isolation 
*  Phil.  Trans.  1868,  p.  555  and  plate  xxxiii. 
