﻿942 
  Prof. 
  R. 
  W. 
  Wood 
  on 
  a 
  Method 
  of 
  Showing 
  

  

  possible 
  that 
  the 
  experimenters 
  who 
  have 
  observed 
  it 
  by 
  

   other 
  methods 
  may 
  be 
  able 
  to 
  pick 
  it 
  up 
  by 
  this 
  method, 
  as 
  I 
  

   may 
  have 
  worked 
  with 
  too 
  strong- 
  or 
  too 
  feeble 
  illuminations, 
  

   though 
  I 
  have 
  varied 
  them 
  over 
  wide 
  limits. 
  If 
  it 
  can 
  be 
  

   obtained 
  by 
  this 
  method, 
  all 
  doubt 
  as 
  to 
  its 
  existence 
  must 
  

   cease, 
  for 
  observation 
  errors 
  are 
  eliminated, 
  and 
  we 
  can 
  

   actually 
  see 
  the 
  increase 
  in 
  the 
  absorbing 
  power. 
  

  

  Obviously 
  uranium 
  glass 
  is 
  not 
  well 
  suited, 
  for 
  the 
  fluor- 
  

   escence, 
  or 
  rather 
  phosphorescence, 
  of 
  this 
  substance 
  persists 
  

   for 
  an 
  appreciable 
  time 
  after 
  the 
  stimulating 
  radiations 
  are 
  

   cut 
  off. 
  This 
  LS 
  not 
  true, 
  however, 
  for 
  solutions, 
  which 
  show 
  

   no 
  trace 
  of 
  continued 
  emission, 
  even 
  at 
  the 
  highest 
  speeds 
  of 
  

   the 
  phosphoroscope 
  disks. 
  

  

  Inasmuch 
  as 
  variations 
  in 
  the 
  intensity 
  of 
  the 
  source, 
  

   provided 
  they 
  are 
  do! 
  too 
  rapid, 
  give 
  no 
  trouble 
  in 
  the 
  case 
  

   of 
  this 
  method, 
  the 
  arc 
  has 
  beeu 
  u>^d 
  in 
  all 
  of 
  the 
  work. 
  It 
  

   was 
  Pound 
  thai 
  a 
  220 
  volt 
  arc, 
  with 
  a 
  heavy 
  ballast 
  resistance 
  

   in 
  circuit, 
  gave 
  excellent 
  results, 
  the 
  hissing 
  and 
  sputtering 
  

   bo 
  common 
  with 
  the 
  1 
  1" 
  vol! 
  currenl 
  being 
  absent. 
  

  

  The 
  light 
  from 
  the 
  arc 
  was 
  focussed 
  upon 
  the 
  perforated 
  

   disk 
  by 
  means 
  of 
  two 
  short-focus 
  lenses, 
  one 
  fixed 
  in 
  position, 
  

   the 
  other 
  arranged 
  so 
  that 
  it 
  could 
  be 
  rotated 
  through 
  a 
  

   small 
  arc 
  about 
  an 
  axis 
  coincident 
  with 
  the 
  axis 
  of 
  the 
  re- 
  

   volving 
  disk, 
  the 
  length 
  of 
  the 
  supporting 
  arm 
  being 
  equal 
  

   to 
  the 
  radius 
  of 
  the 
  disk. 
  The 
  distance 
  through 
  which 
  the 
  

   lens 
  could 
  be 
  moved 
  was 
  regulated 
  by 
  two 
  stops, 
  so 
  adjusted 
  

   that 
  in 
  one 
  position 
  tie- 
  two 
  image- 
  of 
  the 
  arc 
  fell 
  upon 
  the 
  

   opening- 
  ot 
  the 
  disk 
  simultaneously, 
  while 
  in 
  the 
  other 
  

   position 
  the 
  image 
  Formed 
  by 
  one 
  lens 
  fell 
  midway 
  between 
  

   two 
  of 
  the 
  opening- 
  when 
  the 
  other 
  was 
  illuminated. 
  The 
  

   disk 
  was 
  mounted 
  on 
  the 
  shaft 
  of 
  a 
  small 
  electric 
  motor 
  

   operated 
  at 
  very 
  slow 
  speed. 
  Giass 
  tanks 
  filled 
  with 
  a 
  very 
  

   dilute 
  solution 
  of 
  fluorescein 
  were 
  mounted 
  behind 
  the 
  open- 
  

   in 
  such 
  a 
  position 
  that 
  they 
  received 
  the 
  flashes 
  of 
  light 
  

   transmitted 
  by 
  the 
  disk. 
  By 
  this 
  arrangement 
  we 
  can 
  stud} 
  r 
  

   the 
  fluorescent 
  absorption 
  for 
  the 
  light 
  emitted 
  by 
  the 
  fluor- 
  

   escent 
  substance, 
  the 
  case 
  being 
  analogous 
  to 
  that 
  studied 
  

   bv 
  Burke. 
  The 
  movable 
  lens 
  could 
  be 
  moved 
  rapidly 
  back 
  

   and 
  forth, 
  but 
  no 
  flicker 
  in 
  the 
  intensity 
  of 
  light 
  was 
  observed. 
  

   Diaphragms 
  were 
  introduced 
  to 
  restrict 
  the 
  field, 
  and 
  the 
  

   relative 
  intensities 
  were 
  varied, 
  but 
  the 
  effect 
  could 
  not 
  be 
  

   found. 
  To 
  render 
  the 
  change 
  still 
  more 
  marked, 
  if 
  it 
  existed, 
  

   a 
  nicol 
  prism 
  was 
  placed 
  between 
  the 
  two 
  tanks, 
  and 
  a 
  second 
  

   nicol, 
  with 
  its 
  polarizing 
  plane 
  parallel 
  to 
  that 
  of 
  the 
  first, 
  

   placed 
  in 
  front 
  of 
  the 
  eye. 
  By 
  this 
  arrangement 
  we 
  can 
  

   reduce 
  the 
  intensity 
  of 
  the 
  light 
  coming 
  from 
  the 
  fluorescing 
  

  

  