﻿Transmission 
  Fixed-Arm 
  Spectroscopes. 
  

  

  457 
  

  

  half 
  the 
  breadth 
  of 
  the 
  refracting 
  prism, 
  and 
  is 
  exactly 
  

   opposite 
  to 
  the 
  full 
  upper 
  half 
  of 
  it. 
  The 
  prism 
  Im 
  is 
  of 
  the 
  

  

  Fig. 
  10. 
  

  

  / 
  

  

  CL- 
  

  

  / 
  

  

  1 
  

  

  — 
  • 
  is 
  

  

  <-- 
  

  

  

  ' 
  > 
  

  

  > 
  _ 
  

  

  = 
  V 
  

  

  V- 
  

  

  

  — 
  ' 
  « 
  . 
  

  

  " 
  7™* 
  

  

  \ 
  

  

  

  1 
  , 
  < 
  

  

  

  

  * 
  

  

  

  

  T 
  — 
  <r 
  i 
  

  

  

  ir 
  

  

  

  Ficr. 
  11. 
  

  

  A 
  

  

  

  

  I 
  

  

  

  

  *r 
  ^-""""\ 
  

  

  

  

  

  <# 
  

  

  \. 
  

  

  •v 
  

  

  

  c 
  

  

  — 
  < 
  — 
  ■ 
  

  

  ^ 
  \ 
  

  

  A^A 
  < 
  

  

  

  

  

  same 
  size 
  as 
  fh, 
  but 
  has 
  its 
  lower 
  edge 
  at 
  a 
  lower 
  level 
  than 
  

   the 
  bottom 
  of 
  the 
  refracting 
  prism, 
  so 
  that 
  a 
  space 
  is 
  left 
  

   between 
  the 
  reflecting 
  prisms. 
  A 
  beam 
  of 
  light 
  from 
  the 
  

   collimator 
  passes 
  from 
  C 
  between 
  the 
  reflecting 
  prisms, 
  is 
  

   refracted 
  and 
  reflected 
  at 
  the 
  half-prism, 
  and 
  so 
  passes 
  to 
  

   and 
  fro, 
  taking 
  the 
  course 
  indicated 
  by 
  the 
  arrows 
  in 
  figs. 
  10 
  

   and 
  11, 
  and 
  finally 
  passes 
  from 
  the 
  reflector 
  Im 
  under 
  the 
  

   refracting 
  prism 
  into 
  the 
  observing-telescope 
  at 
  T, 
  The 
  

   result 
  in 
  dispersing 
  and 
  resolving 
  power 
  is 
  equivalent 
  to 
  

   passage 
  through 
  a 
  train 
  of 
  as 
  many 
  complete 
  prisms 
  as 
  there 
  

   are 
  to-and-fro 
  journeys 
  of 
  the 
  beam. 
  

  

  This 
  arrangement 
  is 
  better 
  than 
  I. 
  and 
  II., 
  because 
  less 
  

   lio-ht 
  is 
  lost 
  by 
  reflexion; 
  and 
  it 
  is 
  better 
  than 
  III. 
  because 
  it 
  

   gives 
  full 
  resolving 
  power. 
  Moreover, 
  the 
  beam 
  passes 
  

   through 
  the 
  refracting 
  prism 
  in 
  a 
  principal 
  plane, 
  so 
  that 
  all 
  

   the 
  prisms 
  may 
  be 
  placed 
  quite 
  close 
  together, 
  and 
  the 
  

   apparatus 
  thus 
  made 
  more 
  compact. 
  The 
  number 
  of 
  trans- 
  

   missions 
  possible 
  depends 
  upon 
  the 
  width 
  of 
  the 
  opening 
  left 
  

   between 
  the 
  reflecting 
  prisms 
  fh 
  aud 
  Im. 
  A 
  very 
  large 
  

   number 
  of 
  transmissions 
  may 
  be 
  used 
  if 
  this 
  opening 
  is 
  made 
  

   small. 
  Another 
  possible 
  form 
  would 
  be 
  to 
  make 
  the 
  prisms 
  

   fh 
  and 
  I 
  m 
  also 
  refract, 
  by 
  giving 
  them 
  the 
  form 
  produced 
  by 
  

   cutting 
  the 
  prism 
  of 
  fig. 
  9 
  by 
  a 
  plane 
  through 
  e 
  at 
  right 
  angles 
  

   to 
  the 
  face 
  abed. 
  

  

  Phil. 
  Mag. 
  S. 
  6. 
  Vol. 
  3. 
  No. 
  16. 
  April 
  1902. 
  2 
  H 
  

  

  