﻿98 
  Prof. 
  John 
  Trowbridge 
  on 
  

  

  edge 
  is 
  nearly 
  on 
  the 
  horizontal 
  axis 
  of 
  the 
  mirror. 
  Either 
  

   S 
  or 
  P 
  is 
  shifted 
  nearer 
  to 
  or 
  further 
  from 
  M, 
  until 
  the 
  image 
  

   of 
  S 
  in 
  P 
  coincides 
  with 
  the 
  image 
  of 
  S 
  in 
  M, 
  whether 
  viewed 
  

  

  Fig. 
  4. 
  

  

  E' 
  

  

  from 
  E 
  or 
  from 
  E'. 
  Under 
  these 
  circumstances 
  it 
  = 
  a 
  + 
  b, 
  

   v 
  = 
  a 
  — 
  b, 
  whence 
  r 
  can 
  be 
  obtained. 
  As 
  the 
  image 
  formed 
  

   by 
  reflexion 
  in 
  M 
  is 
  smaller 
  than 
  that 
  formed 
  in 
  P, 
  it 
  is 
  

   advisable 
  to 
  use 
  a 
  very 
  narrow 
  slit 
  at 
  S. 
  

  

  To 
  determine 
  the 
  focal 
  length 
  of 
  a 
  concave 
  lens, 
  the 
  mirror 
  

   is 
  removed, 
  the 
  concave 
  lens 
  is 
  placed 
  so 
  that 
  its 
  face 
  touches 
  

   the 
  silvered 
  back 
  of 
  P, 
  and 
  is 
  half 
  covered 
  by 
  it. 
  A 
  stout 
  

   vertical 
  rod 
  is 
  placed 
  at 
  some 
  considerable 
  distance 
  behind 
  the 
  

   mirror 
  on 
  the 
  prolongation 
  of 
  the 
  axis 
  of 
  the 
  lens, 
  and 
  the 
  pin 
  

   S 
  is 
  shifted 
  to 
  or 
  from 
  the 
  mirror 
  until 
  the 
  image 
  of 
  the 
  pin 
  

   seen 
  by 
  reflexion 
  in 
  the 
  plane 
  mirror 
  coincides 
  with 
  the 
  

   image 
  of 
  the 
  rod 
  as 
  seen 
  directly 
  through 
  the 
  lens, 
  whether 
  

   we 
  look 
  from 
  E 
  or 
  from 
  E'. 
  This 
  gives 
  the 
  position 
  of 
  the 
  

   virtual 
  image 
  of 
  the 
  rod 
  ; 
  whence 
  the 
  focal 
  length 
  can 
  be 
  

   calculated. 
  

  

  The 
  pin 
  or 
  rod 
  can 
  with 
  advantage 
  be 
  replaced 
  by 
  a 
  vertical 
  

   straight 
  edge 
  well 
  illuminated. 
  

  

  Both 
  these 
  methods 
  will 
  be 
  found 
  very 
  satisfactory 
  for 
  

   elementary 
  students. 
  The 
  apparatus 
  is 
  very 
  simple, 
  and 
  the 
  

   method 
  seems 
  to 
  me 
  instructive 
  in 
  illustrating 
  the 
  subject 
  of 
  

   parallax. 
  

  

  VIII. 
  High 
  Electromotive 
  Force. 
  By 
  John 
  Trowbridge*. 
  

  

  LORD 
  KELVIN, 
  in 
  a 
  paper 
  dated 
  April 
  12, 
  1860f, 
  entitled 
  

   " 
  Measurement 
  of 
  Electromotive 
  Force 
  Required 
  to 
  

   Produce 
  a 
  Spark," 
  states 
  that 
  "there 
  is 
  a 
  much 
  less 
  rapid 
  

  

  * 
  Communicated 
  by 
  the 
  Author. 
  

  

  f 
  ' 
  Papers 
  on 
  Electrostatics 
  and 
  Magnetism/ 
  Macmillan 
  : 
  London, 
  

   1872. 
  P. 
  258. 
  

  

  