﻿Transmission 
  Fixed- 
  Arm 
  Spectroscopes, 
  

  

  Vol 
  

  

  This 
  is 
  done 
  by 
  supporting 
  the 
  prisms 
  on 
  disks 
  with 
  cogged 
  

   edges, 
  represented 
  by 
  the 
  circles 
  surrounding 
  them 
  in 
  fig. 
  1, 
  

   and 
  having 
  a 
  rod 
  XY 
  with 
  tangent-screws 
  pressed 
  against 
  

   these 
  disks 
  by 
  springs 
  A, 
  k. 
  The 
  rod 
  X 
  Y 
  can 
  be 
  turned 
  

   about 
  the 
  point 
  m 
  so 
  as 
  to 
  remove 
  the 
  tangent-screws 
  from 
  

   the 
  disks 
  for 
  a 
  quick 
  motion 
  o£ 
  the 
  prisms. 
  The 
  prisms 
  are 
  

   attached 
  to 
  the 
  disks 
  by 
  mountings 
  which 
  admit 
  of 
  their 
  

   accurate 
  adjustment. 
  When 
  the 
  instrument 
  has 
  been 
  adjusted 
  

   for 
  light 
  of 
  one 
  refrangibility, 
  it 
  is 
  adjusted 
  for 
  all 
  refran- 
  

   gibilities 
  ; 
  and 
  the 
  whole 
  spectrum 
  can 
  be 
  made 
  to 
  cross 
  

   the 
  field 
  ot 
  view 
  by 
  turning 
  the 
  head 
  of 
  the 
  tangent- 
  

   screws. 
  

  

  The 
  mounting 
  of 
  a 
  prism 
  is 
  shown 
  in 
  fig. 
  3. 
  The 
  prism 
  is 
  

  

  Fig. 
  3. 
  

  

  fixed 
  in 
  a 
  brass 
  cell 
  X 
  which 
  is 
  attached 
  by 
  springs 
  and 
  screws 
  

   to 
  the 
  upright 
  plate 
  P. 
  From 
  the 
  back 
  of 
  the 
  prism-cell 
  a 
  

   rod 
  r 
  passes 
  freely 
  through 
  a 
  hole 
  in 
  the 
  plate 
  P, 
  and 
  is 
  sur- 
  

   rounded 
  by 
  a 
  spiral 
  spring 
  which 
  presses 
  the 
  cell 
  against 
  the 
  

   fixed 
  point 
  x 
  (which 
  enters 
  a 
  slight 
  depression 
  in 
  the 
  back 
  

   of 
  the 
  cell) 
  and 
  the 
  points 
  of 
  the 
  screws 
  y 
  and 
  z. 
  The 
  

   cell 
  is 
  pressed 
  against 
  the 
  point 
  of 
  the 
  serew 
  r 
  w 
  by 
  a 
  

   spring 
  on 
  the 
  opposite 
  side. 
  The 
  plate 
  P 
  is 
  supported 
  with 
  

   freedom 
  for 
  adjustment 
  by 
  a 
  bolt 
  and 
  nut 
  passing 
  through 
  

   the 
  slot 
  g. 
  

  

  The 
  breadth 
  of 
  the 
  beam 
  which 
  can 
  be 
  used 
  is 
  limited 
  

  

  