﻿474 
  Farwell 
  — 
  An 
  Optical 
  Bench 
  for 
  Elementary 
  Work. 
  

  

  diagram. 
  This 
  raises 
  the 
  stick 
  to 
  a 
  convenient 
  height 
  above 
  

   the 
  table, 
  where 
  it 
  is 
  ready 
  to 
  receive 
  the 
  fittings. 
  These 
  

   are 
  all 
  attached 
  to 
  the 
  stick 
  by 
  means 
  of 
  the 
  clamp, 
  B, 
  the 
  

   upper 
  part 
  of 
  which 
  is 
  drilled 
  to 
  receive 
  any 
  J-inch 
  stock. 
  

   The 
  face 
  of 
  the 
  clamp 
  is 
  marked 
  in 
  machining 
  with 
  a 
  

   line 
  opposite 
  the 
  axis 
  of 
  the 
  J-inch 
  hole, 
  so 
  that 
  the 
  error 
  in 
  

   locating 
  the 
  index 
  is 
  made 
  very 
  small. 
  The 
  clamp 
  is 
  entirely 
  

   open 
  on 
  the 
  front 
  that 
  it 
  may 
  be 
  machined 
  easily 
  and 
  that 
  

   it 
  may 
  be 
  quickly 
  set 
  on 
  any 
  part 
  of 
  the 
  bar. 
  It 
  was 
  expected 
  

   that 
  there 
  might 
  be 
  some 
  trouble 
  on 
  account 
  of 
  the 
  ease 
  

   with 
  which 
  the 
  clamps 
  might 
  "fall" 
  off 
  the 
  stick, 
  but 
  in 
  

   the 
  two 
  years 
  in 
  which 
  the 
  apparatus 
  has 
  been 
  used 
  there 
  has 
  

   been 
  practically 
  no 
  difficulty. 
  

  

  The 
  design 
  of 
  a 
  lens 
  holder 
  required 
  more 
  attention 
  than 
  all 
  

   the 
  rest, 
  as 
  it 
  was 
  desirable 
  that 
  a 
  single 
  piece 
  should 
  serve 
  to 
  

   hold, 
  firmly 
  and 
  properly 
  centered, 
  any 
  form 
  of 
  lens, 
  mirror 
  

   or 
  screen, 
  and 
  at 
  the 
  same 
  time 
  permit 
  quick 
  adjustment 
  or 
  

   exchange. 
  The 
  frame 
  C 
  meets 
  all 
  the 
  requirements 
  and 
  has 
  

   proved 
  very 
  satisfactory. 
  The 
  J-inch 
  rod 
  below 
  fits 
  into 
  the 
  

   clamp 
  on 
  the 
  bar. 
  The 
  lower 
  part 
  of 
  the 
  frame 
  is 
  milled 
  with 
  

   a 
  Y 
  groove, 
  the 
  full 
  width 
  of 
  the 
  frame. 
  The 
  movable 
  part 
  is 
  

   made 
  from 
  right-angled 
  brass 
  stock, 
  slotted 
  at 
  the 
  ends. 
  

  

  One 
  great 
  advantage 
  of 
  the 
  apparatus 
  is 
  the 
  ease 
  with 
  which 
  

   a 
  frame, 
  holding, 
  say, 
  a 
  ground-glass 
  screen, 
  can 
  be 
  replaced 
  

   in 
  the 
  same 
  position 
  by 
  a 
  J-inch 
  rod 
  bearing 
  a 
  pin 
  to 
  locate 
  

   an 
  image 
  by 
  the 
  parallax 
  method. 
  

  

  The 
  supports, 
  clamps, 
  and 
  frames 
  are 
  all 
  of 
  cast 
  iron, 
  and 
  

   are 
  of 
  such 
  form 
  that 
  they 
  are 
  easily 
  machined. 
  Experience 
  

   has 
  proved 
  that 
  they 
  are 
  not 
  too 
  clumsy 
  for 
  the 
  work 
  in 
  hand, 
  

   and 
  that 
  they 
  are 
  easily 
  adapted 
  to 
  variations 
  in 
  method 
  in 
  

   the 
  standard 
  experiments. 
  

  

  Columbia 
  University, 
  

   April 
  19, 
  1913. 
  

  

  