﻿456 
  

  

  Prof. 
  W. 
  Cassie 
  on 
  Multiple 
  

  

  For 
  flint-glass 
  the 
  angles 
  at 
  d 
  and 
  g 
  do 
  not 
  differ 
  much 
  from 
  

   90°; 
  so 
  that 
  if 
  de 
  = 
  a=gh 
  and 
  the 
  angles 
  at 
  e 
  and 
  li 
  are 
  

   about 
  60°, 
  the 
  difference 
  of 
  the 
  paths 
  for 
  the 
  pair 
  of 
  prisms 
  

  

  is 
  about 
  9 
  . 
  Whereas 
  the 
  same 
  two 
  prisms 
  with 
  the 
  corners 
  

  

  at 
  c 
  and 
  /not 
  cut 
  off, 
  used 
  as 
  reflecting 
  half 
  -prisms 
  in 
  the 
  

   arrangement 
  first 
  described., 
  would 
  give 
  a 
  difference 
  of 
  path 
  

   about 
  1 
  a 
  for 
  rays 
  traversing 
  each 
  once. 
  That 
  is 
  to 
  say, 
  the 
  

   resolving 
  power 
  of 
  such 
  an 
  instrument 
  of 
  type 
  I. 
  would 
  be 
  

   about 
  eight 
  times 
  that 
  of 
  one 
  of 
  type 
  III. 
  

  

  Other 
  combinations 
  involving 
  similar 
  principles 
  will 
  readily 
  

   suggest 
  themselves. 
  But 
  the 
  three 
  described 
  are 
  sufficient 
  to 
  

   illustrate 
  the 
  method. 
  The 
  number 
  of 
  transmissions 
  may 
  by 
  

   increased 
  by 
  lengthening 
  the 
  prisms 
  or 
  mirrors. 
  The 
  length 
  

   of 
  slit 
  that 
  can 
  be 
  used 
  may 
  be 
  increased 
  in 
  the 
  same 
  way. 
  

   In 
  the 
  instruments 
  I 
  have 
  had 
  made 
  for 
  trial 
  the 
  prisms 
  and 
  

   mirrors 
  were 
  5 
  centimetres 
  high 
  and 
  gave 
  very 
  good 
  results. 
  

   They 
  are 
  simple 
  in 
  construction, 
  and 
  the 
  adjustments 
  are 
  easy 
  

   compared 
  with 
  those 
  of 
  a 
  train 
  of 
  prisms. 
  Moreover, 
  by 
  a 
  few 
  

   turns 
  of 
  an 
  adjusting-screw 
  one 
  can 
  change 
  the 
  number 
  of 
  

   transmissions, 
  and 
  with 
  them 
  the 
  degree 
  of 
  dispersion, 
  step 
  by 
  

   step 
  through 
  a 
  wide 
  range. 
  

  

  IV. 
  

  

  [Addition 
  made 
  Dec. 
  6, 
  1901, 
  after 
  the 
  reading 
  of 
  the 
  paper. 
  ,] 
  

  

  Another 
  arrangement 
  in 
  which 
  the 
  reflectors 
  are 
  totally 
  

   reflecting 
  prisms 
  is 
  shown 
  in 
  figs. 
  10 
  and 
  11. 
  

  

  The 
  back 
  of 
  the 
  half-prism 
  instead 
  of 
  being 
  silvered 
  is 
  

   formed 
  into 
  a 
  right-angled 
  totally-reflecting 
  prism 
  with 
  its 
  

   edge 
  at 
  right 
  angles 
  to 
  the 
  refracting 
  edge 
  of 
  the 
  half-prism, 
  

  

  as 
  shown 
  in 
  fig. 
  9. 
  The 
  beam 
  is 
  reflected 
  back 
  to 
  this 
  com- 
  

   bined 
  half-prism 
  and 
  reflector 
  by 
  two 
  right-angled 
  prisms 
  fh 
  

   .and 
  I 
  m, 
  as 
  shown 
  in 
  figs. 
  10 
  and 
  11. 
  The 
  prism 
  fh 
  is 
  exactly 
  

  

  