[ 887  ] 
XXXVI.  On  the  Influence  of  Temperature  on  the  Refraction  of  Light. 
By  the  Rev.  T.  P.  Dale,  M.A.,  F.R.A.S.,  and  Dr.  J.  H.  Gladstone,  F.R.S. 
Eeceived  June  17, — Eead  June  17,  1858. 
Fkaunhofee,  in  his  original  measurements  of  the  fixed  lines  of  the  spectrum,  noted  the 
thermometer,  and  those  who  since  his  time  have  occupied  themselves  with  the  determi- 
nation of  refractive  indices,  must  have  frequently  noticed  that  changes  of  temperature 
afiect  the  amount  of  refraction ; yet  of  the  many  observations  now  on  record,  few  have- 
affixed  to  them  the  temperature  at  which  they  were  made.  Baden  Powell  has  drawn 
out  some  refractive  indices  of  the  same  substance  taken  at  different  temperatures,  but 
remarks  that  the  whole  subject,  both  as  to  the  facts  of  the  case  and  as  to  their  bear- 
ing on  the  theoretical  formulae,  remains  to  be  investigated  *.  Among  his  beautiful 
researches  on  light,  M.  Jamin  has  recently  published  one  or  two  experiments  on  refrac- 
tion, heat,  and  density  f;  but,  with  this  exception,  he  has  left  untouched  the  great  ques- 
tion of  the  influence  of  temperature. 
Some  years  ago  Baden  Powell  kindly  lent  his  instrument  to  one  of  us  for  the  pur- 
pose of  investigating  this  question.  It  is  described  and  figured  in  the  Report  of  the 
British  Association  for  1839.  It  consists  essentially  of  a small  telescope  moving  on  a 
cu’cle  graduated  so  finely,  that  by  means  of  a vernier  and  magnifying  glass  it  may  be 
read  to  10";  to  ensure  accuracy,  there  is  a vernier  at  each  end  of  the  moveable  arm:  in 
front  of  the  telescope  are  appliances  for  fixing  any  solid  prism,  or  a hollow  prism  capable 
of  containing  liquids,  in  such  a position  that  there  may  be  seen  through  it  a ray  of  light 
proceeding  from  a distant  slit.  The  telescope  may  be  moved  till  a fine  wire  fastened  in 
the  field  of  riew  is  brought  to  coincide  with  any  of  Fraunhofee’s  lines  in  the  prismatic 
spectrum  thus  obtained,  and  the  angle  indicated  by  the  position  of  the  telescope  on  the 
graduated  circle  may  be  read  off.  In  our  recent  joint  research,  as  we  desired  to  measure 
the  positions  of  the  extreme  hnes  A and  H,  it  was  necessary  to  work  with  the  full  light 
of  the  sun ; its  mirrored  image,  therefore,  was  kept  behind  the  narrow  slit  by  means  of 
a heliostat,  and  the  eye  was  protected  from  the  glare  of  the  yellow  rays  by  means  of  a 
cobalt-blue  glass.  Liquids  were  experimented  on,  and  their  temperature  was  changed 
by  means  either  of  a spirit-lamp  or  of  a freezing  mixture  round  the  hollow  prism,  on  a 
day  when  there  was  little  or  no  moisture  in  the  atmosphere  to  condense  on  the  cold  glass. 
A thermometer,  which  had  been  corrected  by  a standard  instrument,  was  placed  in  the 
liquid  contained  in  the  hollow  prism.  In  the  experiments  two  practical  difficulties  had 
* See  his  work  on  ‘ The  Undulatory  Theory  as  applied  to  the  Dispersion  of  Light,’  p.  125. 
t Coinptes  Eendus,  29  Dec.  1856,  and  23  Nov.  1857. 
MDCCCLVIII.  5 Z 
