﻿632 
  Mr. 
  A. 
  E. 
  Tutton 
  on 
  the 
  

  

  advantages 
  over 
  the 
  Fizeau 
  apparatus 
  are 
  briefly: 
  (1) 
  The 
  

   employment 
  of 
  a 
  micrometric 
  method 
  of 
  measuring- 
  the 
  

   position 
  and 
  width 
  of 
  the 
  interference-bands; 
  (2) 
  the 
  use 
  

   of 
  autocollimation 
  ; 
  (3) 
  the 
  employment 
  of 
  C 
  hydrogen 
  light 
  

   instead 
  of 
  sodium 
  light, 
  as 
  being 
  more 
  truly 
  monochromatic 
  

   and 
  not 
  subject 
  to 
  secondary 
  interference 
  ; 
  and 
  (4) 
  an 
  arrange- 
  

   ment 
  of 
  the 
  thermal 
  chamber 
  which 
  readily 
  permits 
  an 
  

   extension 
  of 
  the 
  range 
  to 
  an 
  upper 
  limit 
  of 
  120°. 
  

  

  The 
  author's 
  method 
  also 
  renders 
  the 
  observer 
  independent 
  

   of 
  the 
  nature 
  of 
  the 
  surface 
  of 
  the 
  substance 
  investigated, 
  as 
  

   to 
  whether 
  it 
  is 
  polishable 
  or 
  not. 
  This 
  is 
  attained 
  by 
  the 
  

   use 
  of 
  the 
  aluminium 
  compensator, 
  a 
  relatively 
  thick 
  disk 
  of 
  

   aluminium 
  laid 
  on 
  the 
  top 
  of 
  the 
  substance. 
  It 
  is 
  thus 
  the 
  

   upper 
  surface 
  of 
  this 
  disk, 
  and 
  not 
  that 
  of 
  the 
  substance, 
  

   which, 
  together 
  with 
  the 
  under 
  surface 
  of 
  the 
  glass 
  cover- 
  

   disk, 
  reflects 
  the 
  interfering 
  red 
  hydrogen 
  light. 
  As 
  

   aluminium 
  expands 
  about 
  2 
  6 
  times 
  as 
  much 
  as 
  platinum 
  - 
  

   iridium, 
  it 
  is 
  easy 
  to 
  choose 
  a 
  disk, 
  out 
  of 
  a 
  series 
  prepared 
  

   for 
  the 
  purpose, 
  which 
  will 
  roughly 
  compensate 
  for 
  the 
  

   expansion 
  of 
  the 
  lengths 
  of 
  the 
  platinum- 
  iridium 
  screws, 
  

   which 
  project 
  above 
  the 
  table 
  of 
  the 
  same 
  alloy 
  on 
  which 
  the 
  

   substance 
  rests. 
  By 
  so 
  doing, 
  approximately 
  the 
  whole 
  ex- 
  

   pansion 
  of 
  the 
  substance 
  is 
  recorded 
  by 
  the 
  movement 
  of 
  the 
  

   interference-bands, 
  affording 
  at 
  once 
  an 
  idea 
  of 
  its 
  relative 
  

   expansibility, 
  or 
  in 
  the 
  case 
  of 
  contraction 
  indicating 
  the 
  fact. 
  

   The 
  author's 
  value 
  (quoted 
  later) 
  for 
  the 
  expansion 
  of 
  alu- 
  

   minium 
  agrees 
  fairly 
  well 
  with 
  that 
  given 
  by 
  Fizeau, 
  and 
  it 
  

   has 
  been 
  proved 
  that 
  any 
  error 
  introduced 
  by 
  the 
  use 
  of 
  this 
  

   third 
  substance 
  cannot 
  exceed 
  +0"02 
  of 
  an 
  interference-band, 
  

   an 
  amount 
  far 
  less 
  than 
  the 
  differences 
  observed 
  between 
  the 
  

   numbers 
  of 
  bands 
  afforded 
  by 
  successive 
  determinations 
  under 
  

   identical 
  conditions. 
  The 
  aluminium 
  compensator 
  is 
  simply 
  

   invaluable 
  when 
  substances 
  incapable 
  of 
  good 
  polish 
  are 
  being- 
  

   investigated, 
  and 
  "biscuit'" 
  porcelain 
  is 
  eminently 
  such 
  a 
  

   case 
  ; 
  and 
  there 
  is 
  the 
  further 
  advantage 
  in 
  the 
  case 
  of 
  a 
  

   piece 
  of 
  tube, 
  of 
  which 
  the 
  section 
  is 
  only 
  an 
  annulus, 
  that 
  

   there 
  is 
  a 
  large 
  field 
  of 
  bands 
  afforded 
  instead 
  of 
  a 
  mere 
  ring- 
  

   showing 
  parts 
  of 
  bands. 
  The 
  polish 
  taken 
  by 
  aluminium 
  is 
  

   pre-eminently 
  suitable, 
  as 
  the 
  intensity 
  of 
  the 
  reflected 
  light 
  

   is 
  about 
  the 
  same 
  as 
  that 
  from 
  the 
  cover-glass. 
  The 
  com- 
  

   pensators 
  keep 
  unimpaired 
  if 
  stored 
  in 
  a 
  desiccator. 
  As 
  some 
  

   misapprehension 
  has 
  arisen 
  as 
  to 
  the 
  use 
  of 
  the 
  compensator, 
  

   the 
  author 
  wishes 
  to 
  emphasize 
  the 
  fact 
  that 
  no 
  accession 
  of 
  

   accuracy 
  is 
  claimed 
  for 
  the 
  method, 
  and, 
  as 
  has 
  just 
  been 
  

   shown, 
  it 
  may 
  introduce 
  +0'02 
  of 
  error; 
  but 
  as 
  this 
  latter 
  

   is 
  absolutely 
  negligible, 
  the 
  other 
  advantages 
  referred 
  to 
  are 
  

  

  