﻿94: 
  

  

  Prof. 
  W. 
  Stroud 
  on 
  a 
  Tele 
  metrical 
  

  

  convex 
  surface. 
  Oar 
  mirror 
  was 
  silvered 
  at 
  the 
  back, 
  the 
  

   wire 
  resting 
  against 
  the 
  front 
  glass 
  surface. 
  Under 
  these 
  

   circumstances 
  an 
  image 
  of 
  the 
  wire 
  will 
  be 
  formed 
  by 
  re- 
  

   flexion 
  in 
  the 
  surface. 
  The 
  R.F. 
  is 
  adjusted 
  (a) 
  on 
  the 
  wire, 
  

  

  2t 
  

   (b) 
  on 
  its 
  image. 
  If 
  t 
  is 
  the 
  thickness 
  of 
  the 
  glass, 
  — 
  is 
  the 
  

  

  difference 
  between 
  these 
  readings, 
  from 
  which 
  t 
  can 
  be 
  calcu- 
  

   lated 
  with 
  sufficient 
  accuracy. 
  

  

  Measurement 
  (3) 
  is 
  effected 
  most 
  conveniently 
  by 
  shifting 
  

   the 
  rod 
  or 
  slit 
  a 
  measured 
  large 
  distance 
  (say 
  100 
  cm.) 
  

   towards 
  the 
  mirror, 
  so 
  that 
  the 
  R.F. 
  can 
  be 
  brought 
  to 
  bear 
  

   upon 
  it. 
  

  

  The 
  following 
  results 
  were 
  obtained 
  : 
  — 
  

  

  u. 
  

  

  v. 
  

  

  r. 
  

  

  r 
  (corrected). 
  

  

  143-62 
  

  

  26-82 
  

  

  65-50 
  

  

  65-37 
  

  

  128-79 
  

  

  26-09 
  

  

  6544 
  

  

  65 
  31 
  

  

  118-78 
  

  

  25-66 
  

  

  65-46 
  

  

  6533 
  

  

  124-12 
  

  

  25-88 
  

  

  65-39 
  

  

  65-27 
  

  

  11912 
  

  

  25-67 
  

  

  65-44 
  

  

  6531 
  

  

  The 
  last 
  column 
  gives 
  the 
  value 
  for 
  the 
  radius 
  corrected 
  for 
  

   aberration. 
  The 
  radius 
  of 
  the 
  same 
  mirror 
  measured 
  by 
  our 
  

   spherometer 
  gave 
  65*31 
  and 
  65*37 
  cm. 
  It 
  should 
  be 
  re- 
  

   marked 
  (1) 
  that 
  our 
  spherometer 
  has 
  received 
  a 
  good 
  deal 
  of 
  

   rough 
  usage, 
  (2) 
  that 
  the 
  spherometer 
  measures 
  the 
  curvature 
  

   of 
  the 
  external 
  surface, 
  while 
  the 
  telemeter 
  was 
  used 
  to 
  

   measure 
  the 
  curvature 
  of 
  the 
  silvered 
  surface. 
  Still 
  the 
  

   results 
  show 
  that 
  this 
  telemetrical 
  method 
  is 
  nearly 
  as 
  

   accurate 
  as 
  direct 
  spherometry. 
  

  

  The 
  constants 
  of 
  a 
  concave 
  lens 
  were 
  next 
  determined. 
  

   One 
  face 
  was 
  obviously 
  concave, 
  the 
  other 
  looked 
  nearly 
  flat. 
  

   The 
  radius 
  of 
  the 
  concave 
  face 
  was 
  first 
  got 
  in 
  this 
  way 
  : 
  — 
  

   the 
  rod, 
  suitably 
  and 
  strongly 
  illuminated, 
  was 
  placed 
  between 
  

   the 
  R.F. 
  and 
  the 
  concave 
  surface, 
  and 
  roughly 
  adjusted 
  

   so 
  that 
  it 
  and 
  its 
  image 
  were 
  in 
  close 
  proximity 
  near 
  the 
  

   centre 
  of 
  curvature. 
  In 
  other 
  words 
  u 
  and 
  v 
  were 
  made 
  

   nearly 
  equal 
  and 
  the 
  value 
  of 
  each 
  determined. 
  In 
  this 
  case 
  

   we 
  are 
  dealing 
  with 
  a 
  real 
  image 
  and 
  can 
  bring 
  our 
  R.F, 
  as 
  

   near 
  as 
  we 
  please 
  to 
  the 
  image. 
  

  

  Now 
  a 
  little 
  elementary 
  calculation 
  shows 
  that 
  if 
  R 
  be 
  the 
  

  

  