﻿Sfjlierometer 
  and 
  Focometer. 
  93 
  

  

  prisms 
  forming 
  the 
  prime 
  reflectors 
  supported 
  on 
  stands 
  

   provided 
  with 
  levelling-screws. 
  Slots 
  S, 
  S' 
  are 
  provided 
  in 
  

   the 
  frame-piece 
  D 
  through 
  which 
  pass 
  stout 
  pins 
  P, 
  P' 
  pro- 
  

   vided 
  with 
  clamping-screws 
  helow. 
  B, 
  B' 
  are 
  fixed 
  reflectors 
  

   opposite 
  A 
  and 
  A.' 
  respectively. 
  C 
  is 
  an 
  extra 
  reflector 
  

   (made 
  into 
  one 
  hlock 
  with 
  B 
  and 
  B 
  7 
  ) 
  so 
  as 
  to 
  reflect 
  the 
  two 
  

   beams 
  upward 
  to 
  the 
  eye. 
  If 
  now 
  the 
  prime 
  reflectors 
  be 
  

   adjusted 
  for 
  a 
  particular 
  range, 
  the 
  two 
  portions 
  of 
  the 
  object 
  

   viewed 
  will 
  appear 
  in 
  coincidence 
  in 
  the 
  field 
  of 
  view 
  

   furnished 
  by 
  C. 
  If 
  the 
  two 
  portions 
  are 
  not 
  in 
  coincidence 
  

   they 
  may 
  be 
  brought 
  into 
  coincidence 
  by 
  translating 
  the 
  

   range-finder 
  to 
  or 
  from 
  the 
  object 
  or 
  image. 
  The 
  instrument 
  

   w 
  r 
  orks 
  in 
  fact 
  just 
  like 
  a 
  short-focus 
  telescope, 
  the 
  only 
  

   difference 
  being 
  that 
  the 
  correctness 
  of 
  the 
  range 
  is 
  deter- 
  

   mined 
  by 
  observing 
  whether 
  or 
  not 
  the 
  two 
  portions 
  of 
  the 
  

   object 
  viewed 
  are 
  in 
  alignment. 
  

  

  The 
  instrument 
  was 
  used 
  to 
  determine 
  the 
  radius 
  of 
  curva- 
  

   ture 
  of 
  a 
  convex 
  mirror 
  by 
  the 
  ordinary 
  method. 
  It 
  is 
  clear 
  

   that 
  u 
  should 
  be 
  made 
  as 
  large 
  as 
  possible 
  so 
  as 
  to 
  make 
  v 
  

   large. 
  

  

  The 
  object 
  (a 
  thin 
  rod 
  or 
  slit 
  well 
  illuminated 
  at 
  the 
  back) 
  

   w 
  r 
  as 
  placed 
  at 
  the 
  extremity 
  of 
  the 
  bank, 
  and 
  the 
  mirror 
  at 
  the 
  

   other 
  extremity 
  ; 
  the 
  R.F. 
  (range-finder) 
  was 
  brought 
  up 
  to 
  

   read 
  (1) 
  the 
  image 
  of 
  the 
  rod, 
  (2) 
  the 
  surface 
  of 
  the 
  mirror, 
  

   (3) 
  the 
  position 
  of 
  the 
  object. 
  Measurement 
  (1) 
  is 
  first 
  done 
  

   because 
  the 
  R.F. 
  should 
  be 
  adjusted 
  on 
  the 
  image 
  so 
  as 
  to 
  have 
  

   maximum 
  available 
  base-length 
  and 
  minimum 
  range 
  (the 
  

   conditions 
  for 
  maximum 
  accuracy). 
  The 
  portion 
  of 
  the 
  

   mirror 
  used 
  was 
  restricted 
  to 
  10 
  cm. 
  diameter, 
  so 
  as 
  to 
  make 
  

   the 
  measurements 
  comparable 
  with 
  direct 
  measurements 
  

   made 
  with 
  our 
  spherometer, 
  which 
  has 
  its 
  adjustable 
  point 
  

   5 
  cm. 
  distant 
  from 
  each 
  of 
  the 
  fixed 
  legs. 
  The 
  R.F. 
  is 
  then 
  

   brought 
  up 
  to 
  within 
  a 
  few 
  cms. 
  of 
  the 
  mirror 
  and 
  the 
  prime 
  

   reflectors 
  are 
  angled 
  until 
  the 
  images 
  appear 
  nearly 
  in 
  

   coincidence, 
  the 
  prime 
  reflectors 
  are 
  then 
  rigidly 
  fixed 
  and 
  

   the 
  R.F. 
  is 
  moved 
  to 
  and 
  fro 
  along 
  the 
  bank 
  until 
  exact 
  coin- 
  

   cidence 
  has 
  been 
  obtained, 
  when 
  the 
  reading 
  on 
  the 
  bank 
  is 
  

   taken. 
  If 
  the 
  base-length 
  is 
  10 
  cm. 
  and 
  the 
  image 
  about 
  

   70 
  cm. 
  away, 
  the 
  extreme 
  variations 
  cannot 
  amount 
  to 
  1 
  mm. 
  

   and 
  should 
  not 
  amount 
  to 
  more 
  than 
  \ 
  mm. 
  From 
  fig. 
  1 
  it 
  

   will 
  be 
  seen 
  that 
  the 
  R.F. 
  is 
  so 
  constructed 
  that 
  it 
  does 
  not 
  

   itself 
  obstruct 
  the 
  beams 
  forming 
  the 
  two 
  separate 
  images 
  

   provided 
  its 
  distance 
  from 
  the 
  mirror 
  is 
  sufficient 
  (say 
  8 
  or 
  

   10 
  cm.). 
  

  

  Measurement 
  (2) 
  is 
  effected 
  by 
  suspending 
  a 
  weight 
  by 
  a 
  

   very 
  fine 
  wire 
  so 
  that 
  the 
  wire 
  rests 
  in 
  contact 
  with 
  the 
  

  

  