﻿Transmission 
  Fixed-Arm 
  Spectroscopes. 
  455 
  

  

  the 
  face 
  lif 
  of 
  the 
  next 
  prism, 
  is 
  totally 
  reflected 
  at 
  the 
  face 
  

   g 
  h, 
  and 
  refracted 
  out 
  at 
  the 
  f&cofg. 
  The 
  angles 
  and 
  positions 
  

   of 
  the 
  prisms 
  are 
  made 
  such 
  that 
  the 
  beam 
  emerging 
  ivomfg 
  

   is 
  parallel 
  to 
  that 
  entering 
  at 
  c 
  d 
  (except 
  for 
  a 
  slight 
  incli- 
  

   nation 
  upwards). 
  After 
  leaving 
  the 
  two 
  refracting 
  prisms 
  

   the 
  beam 
  enters 
  k, 
  which 
  returns 
  it 
  after 
  two 
  reflexions 
  to 
  

   the 
  face 
  c 
  d 
  in 
  a 
  direction 
  parallel 
  to 
  that 
  in 
  which 
  it 
  first 
  

   entered 
  that 
  face. 
  Thus 
  the 
  beam 
  circulates 
  round 
  the 
  

   system 
  of 
  three 
  prisms, 
  gradually 
  rising 
  until 
  it 
  finally 
  passes 
  

   over 
  the 
  top 
  of 
  the 
  prism 
  cde 
  into 
  the 
  telescope 
  B. 
  For 
  

   adjustment 
  of 
  the 
  refracting 
  prisms 
  they 
  are 
  mounted 
  on 
  

   levelling- 
  tables 
  which 
  are 
  free 
  to 
  turn 
  about 
  centres 
  at 
  p 
  

   and 
  q, 
  and 
  are 
  linked 
  as 
  shown 
  in 
  fig. 
  6 
  to 
  a 
  screw 
  working 
  

   in 
  a 
  fixed 
  block 
  Y, 
  so 
  that 
  by 
  turning 
  the 
  screw 
  the 
  angle 
  

   between 
  the 
  faces 
  c 
  e 
  and 
  fh 
  can 
  be 
  altered 
  while 
  the 
  positions 
  

   of 
  the 
  prisms 
  are 
  kept 
  symmetrical 
  with 
  respect 
  to 
  the 
  central 
  

   line 
  of 
  the 
  system. 
  Thus 
  when 
  the 
  instrument 
  is 
  adjusted 
  for 
  

   one 
  wave-length, 
  the 
  whole 
  spectrum 
  can 
  be 
  brought 
  into 
  

   view 
  by 
  simply 
  turning 
  the 
  adjusting 
  screws. 
  

  

  It 
  is 
  not 
  necessary 
  that 
  the 
  refracting 
  prisms 
  be 
  complete, 
  

   because 
  a 
  ray 
  entering 
  near 
  c 
  strikes 
  the 
  face 
  ec 
  after 
  

   refraction, 
  and 
  is 
  for 
  present 
  purposes 
  lost 
  ; 
  consequently 
  the 
  

   portion 
  shaded 
  in 
  fig. 
  8 
  may 
  be 
  cut 
  from 
  the 
  prisms 
  at 
  the 
  

  

  Fig. 
  8. 
  

  

  corners 
  c 
  and 
  /. 
  If 
  h 
  g 
  m 
  I 
  is 
  the 
  actual 
  form 
  of 
  prism 
  used, 
  

   it 
  is 
  necessary 
  to 
  cut 
  it 
  so 
  that 
  the 
  beam 
  in 
  passing 
  between 
  

   the 
  refracting 
  prisms 
  meets 
  the 
  faces 
  ce 
  and 
  fh 
  as 
  nearly 
  as 
  

   possible 
  at 
  normal 
  incidence 
  ; 
  because 
  if, 
  for 
  instance, 
  the 
  

   prism 
  were 
  made 
  triangular 
  of 
  the 
  form 
  h 
  m 
  g 
  } 
  the 
  refraction 
  

   at 
  the 
  face 
  m 
  h 
  would 
  produce 
  dispersion 
  opposite 
  to 
  that 
  due 
  

   to 
  mg. 
  

  

  This 
  instrument 
  is 
  much 
  inferior 
  to 
  the 
  others 
  in 
  resolving 
  

   power. 
  To 
  get 
  the 
  internal 
  reflexions 
  in 
  the 
  refracting 
  prisms 
  

   total, 
  it 
  is 
  necessary 
  to 
  make 
  the 
  refractions 
  at 
  a 
  large 
  

   obliquity. 
  That 
  gives, 
  no 
  doubt, 
  considerable 
  dispersion. 
  

   But 
  the 
  difference 
  between 
  the 
  longest 
  and 
  shortest 
  paths 
  of 
  

   rays 
  through 
  the 
  refracting 
  prisms 
  is 
  comparatively 
  small. 
  

  

  