or  TEMPEEATUEE  ON  THE  EEFEACTION  OE  LIGHT. 
889 
be  somewhat  different,  the  whole  amount  of  difference  of  refraction  between  0°  C.  and 
42°‘5  C.  being  for  A,  0’0317,  for  D,  0‘0360,  and  for  H,  0-0397.  This  fact  is  exhibited, 
though  under  another  form,  in  the  succeeding  column,  where  the  refractive  index  for  A 
is  subtracted  fi-om  that  for  H ; and  as  these  lines  are  at  the  two  extremities  of  the 
prismatic  image,  represents  the  measureable  length  of  the  spectrum.  It  evidently 
decreases  as  the  temperatm’e  rises. 
The  last  column  gives  the  dispersive  power  at  the  different  temperatures,  that  is,  the 
ratio  between  the  length  of  the  spectrum  and  the  mean  refraction,  or 
The  refractive  indices  of  all  the  larger  lines  at  15°  C.  have  been  thus  determined: — 
A. 
B. 
c. 
D. 
E. 
F. 
G. 
H. 
1-6114 
1-6177 
1-6209 
1-6303 
1-6434 
1-6554 
1-6799 
1-7035 
Water. — Distilled  water,  which  had  been  boiled  to  expel  the  air,  was  examined, 
as  also  Wenham  Lake  ice-water.  The  following  Table  gives  the  average  of  several 
determinations : — 
Tempera- 
ture. 
Refractive 
index  of  A. 
Refractive 
index  of  D. 
Refractive 
index  of  H. 
Sensitiveness 
per  5°  (D). 
Length  of 
spectrum. 
Dispersive 
power. 
0°  c. 
5 
10 
15 
20 
25 
30 
35 
40 
45 
50 
55 
60 
65 
70 
80 
1-3291 
1-3290 
1-3288 
1-3284 
1-3279 
1-3275 
1-3270 
1-3264 
1-3257 
1-3250 
1-3241 
1-3235 
1-3223 
1-32]  8 
1-3203 
1-3178 
1-3330 
1-3329 
1-3327 
1-3324 
1-3320 
1-3317 
1-3309 
1-3303 
1-3297 
1-3288 
1-3280 
1-3271 
1-3259 
1-3249 
1-3237 
1-3438 
1-3436 
1-3434 
1-3431 
1-3427 
1-3420 
1-3415 
1-3410 
1-3405 
1-3396 
1-3388 
1-3380 
1-3367 
1-3344 
1-3321 
0-0001 
0-0002 
0-0003 
0-0004 
0-0003 
0-0006 
0-0006 
0-0006 
0-0009 
0-0008 
0-0009 
0-0012 
0-0010 
0-0012 
0-0012(A) 
0-0147 
0-0146 
0-0146 
0-0147 
0-0148 
0-0145 
• 0-0145 
0-0146 
0-0148 
0-0146 
0*0147 
0-0145 
0-0144 
0-0141 
0-0143 
0-0429 
0-0439 
0-0445 
0-0438 
0-0449 
0-0448 
0-0441 
0-0435 
The  experiments  of  Jamin  were  also  made  with  water.  He  was  desirous  of  deter- 
mining whether  the  index  of  refraction  varied  fari  'passu  with  the  density,  which  it 
should  do  if  the  theory  of  emission  be  the  correct  one ; accordingly  he  gradually  cooled 
dowTi  water  to  the  freezing-point,  and  found  that  the  index  steadily  increased,  passed 
4°  C.,  the  point  of  maximum  density,  and  still  increased  till  0°.  Hence  he  deduced  the 
independence  of  the  two  phenomena,  change  of  density,  and  change  of  refraction  by 
heat.  His  instrument,  being  constructed  so  as  to  take  advantage  of  the  phenomena  of 
interference,  is  more  delicate  than  that  employed  by  us,  but  it  is  more  difficult  to 
obtain  numerical  data  by  it.  Our  determinations  were  performed  repeatedly  and  most 
carefully  on  water  near  the  freezing-point : they  confirm  the  observations  of  the  French 
physicist,  but  show  at  the  same  time  that  the  remarkable  reversion  of  the  change  of 
5 z 2 
