﻿Physics. 
  217 
  

  

  accord 
  was 
  obtained 
  between 
  several 
  series 
  of 
  measures, 
  and 
  the 
  

   following 
  constants 
  were 
  fixed 
  for 
  the 
  refractive 
  indices 
  of 
  rock 
  

   salt 
  and 
  lluorite 
  in 
  air 
  at 
  20° 
  C. 
  and 
  760 
  mm. 
  pressure. 
  Average 
  

   wave-length 
  of 
  radiations, 
  0.7604/x. 
  

  

  For 
  rock 
  salt 
  n 
  = 
  1.536818 
  ± 
  .000009 
  

   For 
  fluorite 
  n 
  — 
  1.431020 
  ± 
  .000006 
  

  

  (6) 
  Do 
  all 
  rock-salt 
  prisms 
  have 
  the 
  same 
  dispersion? 
  We 
  

   were 
  led 
  to 
  believe 
  the 
  affirmative 
  upon 
  this 
  very 
  important 
  

   question 
  by 
  recorded 
  results 
  from 
  many 
  prisms, 
  but 
  we 
  have 
  

   conclusive 
  evidence 
  in 
  the 
  following 
  comparison 
  of 
  the 
  dispersion 
  

   of 
  three 
  salt 
  prisms, 
  two 
  from 
  Russian 
  and 
  one 
  from 
  Bavarian 
  

   salt, 
  between 
  wave-lengths 
  0.4/x, 
  and 
  4.0/x. 
  The 
  results 
  indicated 
  

   the 
  affirmative, 
  for 
  the 
  differences 
  in 
  the 
  refractive 
  indices 
  in 
  all 
  

   this 
  range 
  never 
  exceeded 
  the 
  probable 
  experimental 
  error 
  of 
  

   determination. 
  To 
  be 
  more 
  precise, 
  the 
  results 
  at 
  A 
  were 
  as 
  

   follows 
  : 
  

  

  Prism 
  R. 
  B. 
  I. 
  n 
  = 
  1.536818 
  + 
  .000009 
  

   Prism 
  R. 
  B. 
  II. 
  n 
  — 
  1.536844 
  ± 
  .000006 
  

   Prism 
  S. 
  P. 
  L. 
  T. 
  n 
  = 
  1.536812 
  ± 
  .000005 
  

  

  At 
  other 
  points 
  the 
  differences 
  were 
  of 
  the 
  same 
  order 
  of 
  

   magnitude. 
  

  

  It 
  follows 
  then, 
  as 
  has 
  been 
  anticipated 
  and 
  elsewhere 
  pointed 
  

   out 
  by 
  Langley, 
  that 
  this 
  most 
  interesting 
  crystal, 
  whose 
  optical 
  

   application 
  from 
  the 
  time 
  of 
  Melloni 
  to 
  the 
  commencement 
  of 
  

   these 
  observations 
  has 
  been 
  chiefly 
  qualitative 
  as 
  a 
  transmitter 
  of 
  

   special 
  radiations, 
  can 
  now 
  be 
  used 
  quantitatively 
  with 
  practical 
  

   convenience 
  in 
  the 
  form 
  of 
  a 
  60° 
  prism, 
  as 
  a 
  standard 
  of 
  refrac- 
  

   tion 
  to 
  which 
  all 
  wave-lengths 
  may 
  be 
  referred 
  with 
  the 
  same 
  

   order 
  of 
  precision 
  as 
  to 
  the 
  grating. 
  

  

  (7) 
  The 
  temperature-coefficient 
  of 
  refractive 
  indices 
  for 
  rock- 
  

   salt 
  prisms. 
  — 
  Bolographs 
  were 
  taken 
  at 
  low 
  and 
  high 
  constant 
  

   temperatures, 
  and 
  from 
  these 
  in 
  connection 
  with 
  former 
  results 
  

   the 
  temperature 
  coefficient 
  for 
  the 
  whole 
  range 
  of 
  radiations 
  

   covered 
  by 
  our 
  holographs 
  was 
  accurately 
  determined. 
  

  

  (8) 
  Comparison 
  of 
  the 
  efficiency 
  of 
  the 
  bolometer 
  and 
  ther- 
  

   mopile. 
  — 
  It 
  will 
  be 
  recalled 
  that 
  the 
  thermopile 
  has 
  recently 
  been 
  

   made 
  far 
  more 
  delicate 
  and 
  efficient 
  by 
  improvements 
  of 
  Rubens, 
  

   so 
  that 
  with 
  him 
  and 
  with 
  some 
  others 
  it 
  has 
  displaced 
  the 
  bolo- 
  

   meter 
  for 
  radiation 
  work. 
  A 
  comparison 
  made 
  here 
  between 
  one 
  

   of 
  these 
  instruments 
  and 
  our 
  bolometer, 
  No. 
  20, 
  shows 
  the 
  latter, 
  

   though 
  of 
  only 
  one-fifteenth 
  the 
  surface, 
  to 
  give 
  twice 
  the 
  deflec- 
  

   tion 
  at 
  the 
  galvanometer 
  when 
  substituted 
  for 
  the 
  thermopile. 
  

   The 
  galvanometer 
  was 
  besides 
  more 
  free 
  from 
  "drift" 
  and 
  

   "wiggle" 
  with 
  the 
  bolometer, 
  and 
  there 
  was 
  no 
  "creep" 
  to 
  the 
  

   deflection 
  with 
  it, 
  while 
  such 
  " 
  creep 
  " 
  lasted 
  5 
  or 
  10 
  seconds 
  

   with 
  the 
  thermopile. 
  The 
  bolometer 
  has 
  besides 
  the 
  advantage 
  

   that 
  it 
  can 
  be 
  made 
  more 
  strictly 
  linear 
  and 
  far 
  narrower 
  than 
  

  

  