888 
EEV.  T.  P.  DALE  AJN^D  DE.  J.  H.  GLADSTONE  ON  THE  INTEXTENCE 
always  to  be  contended  with : the  one  was  the  proper  fixing  of  the  prism,  so  that  its  edge 
should  be  parallel  with  the  slit,  and  that  at  the  same  time  it  should  be  kept  in  such  a 
position  as  to  indicate  the  minimum  deviation.  It  was  found  necessary  to  fix  this 
position  at  the  commencement  of  the  experiment,  and  to  retain  the  same  through  all 
variations  of  temperature,  though  the  angle  of  minimum  deviation  must  vary  slightly 
with  the  change  of  refraction;  not,  however,  in  the  vast  majority  of  cases,  to  an  appre- 
ciable extent.  A more  serious  difficulty  was  that  of  determining  the  precise  tempera- 
ture at  which  an  observation  was  made,  especially  when  the  temperature  of  the  liquid 
was  much  lower  or  higher  than  that  of  the  surrounding  ah’  or  brass-work ; but  it  was  to 
a great  extent  overcome  by  stirring  the  liquid  in  the  prism  with  the  immersed  thermo- 
meter-bulb  just  before  taking  a measurement ; and  this  had  the  additional  advantage  of 
bringing  out  the  lines  more  distinctly ; for  where  a liquid  is  ver-y  sensitive  to  heat  and 
but  little  dispersive,  as  is  the  case  with  ether,  the  spectrum  becomes  too  misty  to  see 
the  lines  unless  the  liquid  be  uniform  in  its  temperature.  A plan  adopted  in  order  to 
obtain  observations  on  two  or  more  lines  at  precisely  the  same  temperature,  was  to  bring 
the  wire  of  the  telescope  exactly  over  one  line,  read  the  angle,  and  while  it  still  coin- 
cided, or  when  rrext  it  did  so,  move  it  suddenly  till  it  came  over  the  other  line. 
The  substances  more  or  less  fully  examined  have  beerr  bisulphide  of  carbon ; water ; 
ether ; methylic,  virric,  amylic  arrd  caprylic  alcohols ; hydrate  of  phenyl,  and  hydrate  of 
cresyl,  the  two  principal  constituents  of  creasote ; phosphorus ; oil  of  cassia ; and  cam- 
phor dissolved  in  alcohol. 
Bisuljphide  of  Carbon. — The  following  Table  gives  the  mean  results  of  many  observa- 
tions made  on  pure  colourless  bisulphide  of  carbon,  boiling  at  43°  C. 
Tempera- 
ture. 
Refractive  1 Refractive 
index  of  A. 'index  of  D. 
Refraetive 
index  of  H. 
Difference 
per  5°  (D). 
Length  of 
spectrum. 
Dispersive 
power. 
o°c. 
5 
10 
15 
20 
25 
30 
35 
40 
42*5 
1-6217 
1-6180 
1-6144 
1-6114 
1-6076 
1-6036 
1-5995 
1-5956 
1-5919 
1-5900 
1-6442 
1-6397 
1-6346 
1-6303 
1-6261 
1-6220 
1-6182 
1-6140 
1-6103 
1-6082 
1-7175 
1-7119 
1-7081 
1-7035 
1-6993 
1-6942 
1-6896 
1-6850 
1-6810 
1-6778 
0-0045 
0-0051 
0-0043 
0-0042 
0-0041 
0-0038 
0-0042 
0-0037 
0-0042 
0-0958 
0-0939 
0-0937 
0-0921 
0-0917 
0-0906 
0-0901 
0-0894 
0-0891 
0-0878 
0-1487 
0-1469 
0-1477 
0-1462 
0-1463 
0-1460 
0-1457 
0-1456 
0-1459 
0-1443 
The  three  columns  that  give  the  refractive  indices  of  the  fixed  lines  of  the  spectrum 
A,  D,  and  H respectively,  speak  for  themselves. 
The  next  column  represents  the  amount  of  change  of  refi’active  index  w’hich  the 
substance  exhibits,  for  each  5°  of  temperature.  For  this  change  the  term  sensitiveness  is 
proposed.  The  difference  is  evidently  uniform,  or  nearly  so,  the  irregularities  being 
within  the  limits  of  errors  of  observation.  It  is  reckoned  from  the  indices  of  the  line  D, 
as  that  line  very  nearly  represents  the  point  of  mean  refraction  in  the  spectrum,  and  a 
glance  will  suffice  to  show  that  were  it  reckoned  for  either  A or  H,  the  average  w’ould 
