﻿452 
  Prof. 
  W. 
  Cassie 
  on 
  Multiple 
  

  

  horizontally 
  only 
  by 
  the 
  breadth 
  of 
  the 
  face 
  of 
  the 
  prism 
  ; 
  but 
  

   vertically 
  the 
  beam 
  must 
  be 
  narrow 
  if 
  a 
  number 
  of 
  reflexions 
  

   are 
  required. 
  Thus 
  the 
  resolving 
  power 
  of 
  the 
  instrument 
  

   depending, 
  as 
  it 
  does, 
  on 
  the 
  horizontal 
  breadth 
  of 
  the 
  beam, 
  

   is 
  not 
  affected 
  by 
  this 
  restriction. 
  The 
  resolving 
  power 
  is 
  

   that 
  of 
  a 
  train 
  of 
  as 
  many 
  complete 
  prisms 
  as 
  there 
  are 
  

   reflexions 
  in 
  the 
  half- 
  prisms. 
  

  

  The 
  beam 
  does 
  not 
  traverse 
  the 
  prisms 
  exactly 
  in 
  a 
  prin- 
  

   cipal 
  plane. 
  But 
  the 
  smallness 
  of 
  this 
  obliquity 
  and 
  the 
  

   narrowness 
  of 
  the 
  beam 
  in 
  a 
  vertical 
  direction 
  make 
  any 
  such 
  

   effect 
  of 
  small 
  consequence. 
  Moreover, 
  the 
  obliquity 
  can 
  

   be 
  indefinitely 
  diminished 
  by 
  increasing 
  the 
  distance 
  between 
  

   the 
  prisms. 
  

  

  II. 
  

  

  Another 
  form 
  of 
  instrument 
  is 
  illustrated 
  in 
  plan 
  in 
  fig. 
  4 
  

   and 
  in 
  elevation 
  in 
  fig. 
  5. 
  In 
  this 
  case 
  one 
  prism 
  and 
  two 
  

   plane 
  mirrors 
  are 
  used. 
  

  

  A 
  is 
  the 
  collimator 
  and 
  B 
  the 
  telescope. 
  C 
  is 
  the 
  prism 
  

   mounted 
  on 
  a 
  levelling-table 
  which 
  rests 
  on 
  and 
  turns 
  with 
  

   the 
  arm 
  Cf. 
  d 
  is 
  a 
  plane 
  mirror 
  fixed 
  above 
  the 
  object-glass 
  

   of 
  the 
  collimator 
  and 
  facing 
  towards 
  the 
  prism, 
  e 
  is 
  a 
  plane 
  

   mirror 
  supported 
  on 
  the 
  extremity 
  of 
  the 
  arm 
  G 
  e, 
  and 
  also 
  

   facing 
  towards 
  the 
  prism. 
  The 
  arm 
  C 
  e 
  is 
  free 
  to 
  turn 
  round 
  

   the 
  centre 
  of 
  the 
  table 
  of 
  the 
  instrument. 
  To 
  the 
  arm 
  Cf 
  is 
  

   attached 
  a 
  screw 
  carrying 
  a 
  block 
  Z, 
  which 
  is 
  free 
  to 
  move 
  

   along 
  the 
  arm, 
  and 
  is 
  linked 
  to 
  a 
  pin 
  X 
  fixed 
  on 
  the 
  table 
  of 
  

   the 
  instrument 
  and 
  to 
  a 
  pin 
  Y 
  on 
  the 
  arm 
  C 
  e. 
  To 
  use 
  the 
  

   prism 
  with 
  minimum 
  deviation 
  Z 
  X 
  and 
  Z 
  Y 
  are 
  made 
  equal, 
  

   and 
  the 
  prism 
  and 
  the 
  mirror 
  e 
  are 
  moved 
  simultaneously 
  by 
  

   turning 
  the 
  screw. 
  

  

  The 
  beam 
  of 
  light 
  emerging 
  from 
  the 
  collimator 
  A 
  passes 
  

   under 
  the 
  mirror 
  d 
  and 
  is 
  deviated 
  by 
  the 
  prism. 
  The 
  

   mirror 
  e 
  is 
  so 
  placed 
  that 
  the 
  light 
  strikes 
  it 
  near 
  its 
  lower 
  

   edge 
  and 
  goes 
  back 
  through 
  the 
  prism. 
  The 
  beam 
  being 
  

   inclined 
  slightly 
  upwards, 
  now 
  strikes 
  the 
  mirror 
  d, 
  and 
  so 
  is 
  

   reflected 
  backwards 
  and 
  forwards 
  between 
  the 
  mirrors 
  tra- 
  

   versing 
  the 
  prism 
  at 
  each 
  journey 
  and 
  gradually 
  climbing 
  

   upwards 
  until 
  finally 
  it 
  passes 
  from 
  the 
  mirror 
  d 
  over 
  the 
  top 
  

   of 
  the 
  prism 
  into 
  the 
  telescope 
  B. 
  If 
  the 
  prism 
  and 
  mirrors 
  

   are 
  once 
  properly 
  adjusted 
  for 
  minimum 
  deviation 
  for 
  light 
  

   of 
  one 
  ref 
  rangibility, 
  they 
  are 
  brought 
  in 
  adjustment 
  into 
  

   the 
  position 
  for 
  any 
  other 
  refrangibility 
  simply 
  by 
  turning 
  

   the 
  head 
  of 
  the 
  screw 
  Cf. 
  

  

  