﻿Splierometer 
  and 
  Focometer. 
  97 
  

  

  posite 
  block 
  of 
  prisms 
  B, 
  W, 
  and 
  C 
  (figs. 
  1 
  and 
  2) 
  is 
  now 
  

   replaced 
  by 
  a 
  single 
  right-angled 
  polished 
  speculum-prism 
  C 
  

   turned 
  edge 
  upwards, 
  as 
  shown 
  in 
  the 
  plan. 
  As 
  it 
  stands, 
  

   this 
  instrument 
  would 
  not 
  be 
  a 
  range-finder 
  at 
  all, 
  but 
  an 
  

   azimuth 
  indicator. 
  A 
  right-angled 
  glass 
  prism 
  P 
  is, 
  however, 
  

   mounted 
  on 
  the 
  stage 
  suppoiting 
  the 
  reflector 
  A', 
  so 
  that 
  the 
  

   beam 
  of 
  light 
  is 
  reversed 
  right 
  for 
  left 
  before 
  falling 
  on 
  A'. 
  

   This 
  converts 
  the 
  instrument 
  into 
  a 
  range-finder. 
  The 
  kind 
  

   of 
  appearance 
  that 
  is 
  presented 
  in 
  the 
  instrument 
  before 
  

   correct 
  alignment 
  has 
  been 
  attained 
  is 
  shown 
  by 
  the 
  lines 
  /, 
  V, 
  

   as 
  seen 
  in 
  the 
  prism 
  C/. 
  Here 
  the 
  object 
  being 
  observed 
  is 
  a 
  

   line 
  perpendicular 
  to 
  the 
  plane 
  of 
  the 
  figure 
  at 
  some 
  distance 
  

   away 
  in 
  the 
  direction 
  of 
  the 
  arrow. 
  A 
  moment's 
  reflection 
  

   will 
  show 
  that 
  the 
  R.F. 
  is 
  too 
  near 
  the 
  object. 
  To 
  produce 
  

   alignment 
  we 
  should 
  require 
  to 
  move 
  the 
  instrument 
  in 
  the 
  

   opposite 
  direction 
  to 
  that 
  of 
  the 
  arrow. 
  The 
  advantage 
  

   which 
  this 
  arrangement 
  presents 
  over 
  the 
  preceding 
  is 
  that 
  

   the 
  speculum-prism 
  0' 
  obstructs 
  very 
  little 
  of 
  the 
  beams. 
  

   Experiment 
  shows 
  that 
  the 
  accuracy 
  obtainable 
  with 
  it 
  is 
  not 
  

   appreciably 
  different 
  from 
  that 
  obtainable 
  with 
  the 
  instrument 
  

   first 
  described. 
  

  

  Conclusions. 
  

  

  1. 
  The 
  telemetrical 
  method 
  described 
  is 
  not 
  much 
  inferior 
  

   to 
  a 
  srjherometer 
  for 
  measuring 
  the 
  radii 
  of 
  curvature 
  of 
  

   convex 
  and 
  approximately 
  plane 
  surfaces. 
  

  

  2. 
  The 
  method 
  is 
  superior 
  in 
  accuracy 
  (though 
  not 
  of 
  

   course 
  in 
  convenience) 
  to 
  the 
  spherometer 
  for 
  measuring 
  the 
  

   radii 
  of 
  curvature 
  of 
  concave 
  surfaces. 
  

  

  3. 
  The 
  method 
  is 
  available 
  for 
  determining 
  all 
  the 
  geo- 
  

   metrical 
  or 
  optical 
  constants 
  of 
  either 
  a 
  convex 
  or 
  a 
  concave 
  

   lens 
  on 
  the 
  optical 
  bank 
  alone. 
  

  

  Appendix. 
  

  

  In 
  connexion 
  with 
  this 
  subject 
  there 
  remains 
  to 
  be 
  described 
  

   a 
  simple 
  apparatus 
  for 
  measuring 
  optically 
  the 
  radius 
  of 
  cur- 
  

   vature 
  of 
  a 
  convex 
  mirror 
  or 
  the 
  focal 
  length 
  of 
  a 
  concave 
  

   lens. 
  

  

  I 
  venture 
  to 
  think 
  that 
  it 
  will 
  be 
  found 
  as 
  useful 
  in 
  ele- 
  

   mentary 
  teaching 
  in 
  other 
  laboratories 
  as 
  it 
  has 
  been 
  found 
  at 
  

   the 
  Yorkshire 
  College. 
  

  

  M 
  is 
  the 
  convex 
  mirror, 
  P 
  is 
  a 
  slip 
  of 
  silvered 
  plate-glass 
  

   (assumed 
  plane), 
  S 
  is 
  a 
  pin 
  well 
  illuminated 
  or 
  narrow 
  slit 
  

   of 
  light. 
  The 
  plate- 
  glass 
  P 
  is 
  arranged 
  so 
  that 
  its 
  upper 
  

  

  Phil. 
  Mag. 
  S. 
  5. 
  Vol. 
  45. 
  No. 
  272. 
  Jan. 
  1898. 
  H 
  

  

  