﻿170 
  ANNUAL 
  EEPORT 
  SMITHSONIAN 
  INSTITUTION, 
  1929 
  

  

  is 
  selective 
  in 
  the 
  sense 
  in 
  which 
  a 
  penny-in-the-slot 
  machine 
  is 
  selec- 
  

   tive 
  ; 
  if 
  we 
  pour 
  radiation 
  of 
  the 
  wrong 
  frequency 
  on 
  to 
  an 
  atom 
  we 
  may 
  

   reproduce 
  the 
  comedy 
  of 
  the 
  millionaire 
  whose 
  total 
  wealth 
  will 
  not 
  

   procure 
  him 
  a 
  box 
  of 
  matches 
  because 
  he 
  has 
  not 
  a 
  loose 
  pennj^, 
  or 
  we 
  

   may 
  reproduce 
  the 
  tragedy 
  of 
  the 
  child 
  who 
  can 
  not 
  obtain 
  a 
  slab 
  of 
  

   chocolate 
  because 
  its 
  hoarded 
  wealth 
  consists 
  of 
  farthings 
  and 
  half- 
  

   pence, 
  but 
  we 
  shall 
  not 
  disturb 
  the 
  hydrogen 
  atom. 
  

  

  This 
  selective 
  action 
  of 
  the 
  atom 
  on 
  radiation 
  is 
  put 
  in 
  evidence 
  in 
  

   a 
  variety 
  of 
  ways, 
  but 
  is 
  perhaps 
  most 
  simply 
  shown 
  in 
  the 
  spectra 
  of 
  

   the 
  stars. 
  Light 
  of 
  all 
  wave 
  lengths 
  streams 
  out 
  from 
  the 
  hot 
  interior 
  

   of 
  a 
  star 
  and 
  bombards 
  the 
  atoms 
  which 
  form 
  its 
  atmosphere. 
  These 
  

   atoms 
  drink 
  up 
  that 
  radiation 
  which 
  is 
  of 
  precisely 
  the 
  right 
  wave 
  

   length, 
  but 
  have 
  no 
  interaction 
  of 
  any 
  kind 
  with 
  the 
  rest, 
  with 
  the 
  

   result 
  that 
  the 
  radiation 
  which 
  is 
  finally 
  emitted 
  from 
  the 
  star 
  is 
  

   deficient 
  in 
  just 
  these 
  particular 
  wave 
  lengths. 
  This 
  is 
  shown 
  by 
  the 
  

   star 
  showing 
  an 
  absorption 
  spectrum 
  of 
  fine 
  lines. 
  As 
  the 
  atoms 
  in 
  the 
  

   star's 
  atmosphere 
  absorb 
  this 
  radiation 
  they 
  move 
  to 
  orbits 
  of 
  higher 
  

   energy, 
  but 
  in 
  course 
  of 
  time 
  they 
  lapse 
  back 
  to 
  their 
  old 
  orbits, 
  and 
  

   in 
  doing 
  so 
  emit 
  energy 
  in 
  the 
  form 
  of 
  radiation 
  of 
  precisely 
  these 
  same 
  

   wave 
  lengths. 
  This 
  does 
  not, 
  as 
  might 
  at 
  first 
  be 
  thought, 
  exactly 
  

   neutralize 
  the 
  absorption 
  of 
  radiation, 
  because 
  the 
  absorbed 
  radiation 
  

   was 
  all 
  traveling 
  outwards, 
  whereas 
  the 
  emitted 
  radiation 
  travels 
  in 
  

   all 
  directions 
  at 
  random. 
  Thus, 
  if 
  we 
  view 
  the 
  atmosphere 
  tangen- 
  

   tially, 
  as 
  we 
  can 
  do 
  with 
  the 
  sun's 
  atmosphere 
  at 
  a 
  total 
  eclipse, 
  we 
  

   observe 
  the 
  same 
  spectrum, 
  no 
  longer 
  as 
  an 
  absorption 
  but 
  as 
  an 
  

   emission 
  spectrum; 
  it 
  no 
  longer 
  consists 
  of 
  dark, 
  but 
  of 
  bright 
  lines 
  — 
  

   the 
  ''flash" 
  spectrum. 
  

  

  Any 
  atom, 
  or 
  indeed 
  any 
  other 
  electrical 
  structure, 
  will 
  select 
  the 
  

   radiation 
  of 
  suitable 
  wave 
  length 
  from 
  all 
  the 
  radiation 
  which 
  falls 
  on 
  

   it, 
  and 
  use 
  the 
  energy 
  of 
  this 
  radiation 
  in 
  rearranging 
  its 
  electron 
  

   orbits. 
  The 
  amount 
  of 
  energy 
  e 
  that 
  the 
  atom 
  absorbs 
  is 
  connected 
  

   with 
  the 
  wave 
  length 
  X 
  of 
  the 
  radiation 
  by 
  the 
  quantum 
  relation 
  

   €K 
  = 
  hC, 
  where 
  h 
  is 
  Planck's 
  constant 
  (6.55 
  X 
  10"^^ 
  erg 
  sec), 
  and 
  C 
  

   is 
  the 
  velocity 
  of 
  light. 
  The 
  quantity 
  e 
  of 
  energy 
  given 
  by 
  this 
  relation 
  

   is 
  called 
  the 
  "quantum" 
  of 
  light 
  of 
  wave 
  length 
  X, 
  and 
  the 
  wave 
  lengths 
  

   of 
  the 
  radiation 
  which 
  any 
  electrical 
  structure 
  selects 
  are 
  determined 
  

   by 
  the 
  condition 
  that 
  the 
  corresponding 
  quantum 
  of 
  eneigy 
  shall 
  

   just 
  suffice 
  to 
  shift 
  its 
  electrons 
  from 
  one 
  orbit 
  to 
  another. 
  Radiation 
  

   will 
  also 
  be 
  absorbed 
  if 
  its 
  quantum 
  provide 
  sufficient 
  energy 
  to 
  tear 
  

   the 
  electron 
  out 
  of 
  the 
  atom 
  altogether, 
  and 
  set 
  it 
  traveling 
  through 
  

   space 
  as 
  a 
  free 
  electron. 
  All 
  radiation 
  of 
  which 
  the 
  wave 
  length 
  is 
  

   less 
  than 
  a 
  certain 
  critical 
  limit 
  fulfils 
  this 
  latter 
  condition. 
  

  

  The 
  more 
  compact 
  an 
  electrical 
  structure 
  is, 
  the 
  greater 
  the 
  energy 
  

   necessary 
  to 
  disturb 
  it; 
  and 
  the 
  greater 
  the 
  quantum 
  of 
  energy 
  e, 
  

  

  