﻿216 
  

  

  ANNUAL 
  EEPOET 
  SMITHSONIAN 
  INSTITUTION, 
  19 
  2 
  9 
  

  

  Ordinary 
  visible 
  light 
  is 
  well 
  represented 
  by 
  the 
  radiation 
  from 
  a 
  

   carbon 
  arc. 
  If 
  its 
  rays 
  are 
  passed 
  through 
  a 
  prism, 
  they 
  are 
  spread 
  

   into 
  a 
  spectrum 
  of 
  many 
  colors, 
  from 
  red 
  to 
  violet, 
  which 
  the 
  prism 
  

   has 
  separated 
  from 
  each 
  other. 
  Beyond 
  the 
  red 
  end 
  of 
  the 
  spectrum 
  

   lie 
  the 
  heat 
  rays. 
  Indeed 
  if 
  we 
  should 
  place 
  a 
  radiometer 
  just 
  beyond 
  

   the 
  red 
  end 
  of 
  the 
  spectrum, 
  we 
  should 
  find 
  it 
  strongly 
  affected 
  by 
  

   the 
  heat 
  rays 
  from 
  the 
  arc. 
  The 
  question 
  arises, 
  are 
  there 
  similar 
  

   radiations 
  beyond 
  the 
  violet 
  which 
  we 
  are 
  unable 
  to 
  see? 
  

  

  If 
  a 
  fluorescent 
  screen 
  of 
  platinum 
  barium 
  cyanide 
  is 
  brought 
  up, 
  

   we 
  notice 
  a 
  brilliant 
  green 
  glow 
  extending 
  far 
  beyond 
  the 
  violet 
  light 
  

   visible 
  on 
  the 
  ordinary 
  screen. 
  Evidently 
  our 
  failure 
  to 
  see 
  light 
  in 
  

   this 
  region 
  is 
  not 
  because 
  there 
  is 
  no 
  light, 
  but 
  because 
  our 
  eyes 
  are 
  

   insensitive 
  to 
  rays 
  of 
  this 
  type. 
  The 
  fluorescent 
  screen 
  changes 
  their 
  

   color 
  so 
  that 
  we 
  can 
  see 
  them. 
  These 
  are 
  the 
  ultra-violet 
  rays, 
  of 
  

  

  CATHODE 
  ELECTRONS 
  

  

  

  Figure 
  1. 
  

  

  -Coolidge 
  X-ray 
  tube. 
  Electrons 
  shot 
  from 
  the 
  cathode 
  against 
  the 
  target 
  produce 
  these 
  

   X 
  rays, 
  which 
  are 
  light 
  of 
  very 
  short 
  wave 
  length 
  

  

  which 
  we 
  have 
  heard 
  so 
  much 
  recently 
  in 
  connection 
  with 
  summer 
  

   sunshine 
  and 
  prevention 
  of 
  rickets. 
  

  

  As 
  one 
  goes 
  farther 
  into 
  the 
  ultra-violet 
  the 
  rays 
  become 
  rapidly 
  

   absorbed 
  by 
  air, 
  and 
  can 
  be 
  studied 
  only 
  in 
  a 
  vacuum. 
  But 
  at 
  still 
  

   shorter 
  wave 
  lengths 
  the 
  rays 
  are 
  again 
  less 
  readily 
  absorbed 
  as 
  we 
  

   approach 
  the 
  region 
  of 
  X 
  rays. 
  A 
  high-tension 
  transformer 
  shoots 
  

   the 
  electrons 
  at 
  high 
  speed 
  from 
  the 
  hot 
  wire 
  cathode 
  against 
  the 
  tung- 
  

   sten 
  target 
  and 
  there 
  X 
  rays 
  are 
  emitted 
  (fig. 
  1). 
  It 
  is 
  like 
  shooting 
  

   a 
  rapid-fire 
  gun 
  at 
  a 
  steel 
  plate. 
  The 
  bullets 
  represent 
  the 
  electrons 
  

   shot 
  from 
  the 
  cathode, 
  and 
  the 
  noise 
  resulting 
  when 
  the 
  bullets 
  bang 
  

   against 
  the 
  plate 
  represents 
  the 
  X 
  rays. 
  

  

  Just 
  as 
  in 
  the 
  case 
  of 
  ultra-violet 
  light, 
  these 
  X 
  rays 
  do 
  not 
  affect 
  

   our 
  eyes. 
  Their 
  existence 
  can, 
  however, 
  be 
  shown 
  by 
  placing 
  in 
  

   their 
  path 
  the 
  same 
  screen 
  as 
  was 
  used 
  to 
  detect 
  the 
  ultra-violet 
  rays. 
  

  

  