﻿386 
  Dr. 
  H. 
  Stansfield 
  on 
  

  

  gives 
  in 
  this 
  point 
  spectrum, 
  therefore, 
  the 
  advantages 
  of 
  a 
  

   very 
  high 
  order 
  without 
  the 
  usual 
  overlapping. 
  

  

  Gehrcke 
  and 
  Baeyer 
  * 
  obtained 
  similar 
  spectra 
  which 
  they 
  

   call 
  interference 
  points, 
  by 
  crossing 
  plane 
  parallel 
  plates, 
  

   and 
  have 
  pointed 
  out 
  the 
  advantages 
  of 
  combining 
  two 
  

   independent 
  high 
  dispersions. 
  

  

  A 
  photograph 
  of 
  the 
  secondary 
  point 
  spectrum, 
  obtained 
  

   with 
  an 
  exposure 
  of 
  one 
  hour, 
  is 
  reproduced 
  in 
  Plate 
  XI., 
  No. 
  3. 
  

   The 
  echelon 
  had 
  the 
  usual 
  tilt, 
  about 
  3°, 
  and 
  the 
  echelon 
  

   table 
  was 
  rotated 
  about 
  2° 
  in 
  the 
  positive 
  direction 
  of 
  6 
  from 
  

   the 
  normal 
  position. 
  The 
  dispersion 
  in 
  the 
  Fabry 
  and 
  Perot 
  

   system 
  is, 
  in 
  this 
  case, 
  twelve 
  times 
  that 
  in 
  the 
  echelon 
  system, 
  

   so 
  the 
  wave-length 
  intervals 
  can 
  be 
  best 
  determined 
  by 
  the 
  

   position 
  of 
  the 
  dots 
  in 
  the 
  former 
  system, 
  while 
  the 
  dots 
  can 
  

   be 
  recognized 
  and 
  their 
  wave-lengths 
  can 
  be 
  roughly 
  fixed 
  

   by 
  their 
  position 
  in 
  the 
  latter 
  system. 
  

  

  There 
  was 
  no 
  difficulty 
  in 
  recognizing 
  the 
  dots 
  marked 
  

   1, 
  2, 
  3, 
  4, 
  5, 
  6, 
  and 
  7 
  in 
  the 
  photograph, 
  which 
  represent 
  

   well-known 
  lines 
  in 
  the 
  green 
  line 
  spectrum. 
  

  

  In 
  order 
  to 
  determine 
  the 
  wave-length 
  intervals, 
  it 
  is 
  

   necessary 
  to 
  calculate 
  A^? 
  the 
  wave-length 
  interval 
  between 
  

   neighbouring 
  orders 
  of 
  the 
  Fabry 
  and 
  Perot 
  spectrum. 
  It 
  was 
  

   found 
  from 
  the 
  formula 
  

  

  Ax 
  = 
  ^^ 
  

  

  ^ 
  d\ 
  

  

  where 
  jo, 
  the 
  order 
  of 
  the 
  spectra, 
  is 
  given 
  by 
  

  

  2 
  lit 
  cos 
  r 
  

  

  T 
  being 
  the 
  angle 
  of 
  refraction 
  of 
  the 
  light 
  in 
  the 
  plates, 
  and 
  

   t 
  their 
  thickness. 
  The 
  value 
  found 
  in 
  this 
  way 
  for 
  the 
  centre 
  

   of 
  the 
  photograph 
  is 
  96*5 
  m.A. 
  The 
  wave-length 
  intervals 
  

   between 
  the 
  components 
  deduced 
  from 
  the 
  measurements 
  of 
  

   the 
  photograph 
  and 
  this 
  value 
  of 
  A^ 
  are 
  given 
  in 
  Table 
  III., 
  

   p. 
  393; 
  most 
  of 
  them 
  agree 
  closely 
  with 
  the 
  values 
  which 
  

   have 
  been 
  found 
  by 
  other 
  methods. 
  

  

  * 
  E. 
  Gehrcke 
  and 
  O. 
  v. 
  Baeyer, 
  Annalen 
  der 
  Physik, 
  xx. 
  p. 
  269 
  

   (1906). 
  

  

  