﻿EFFECTS 
  OF 
  SOLAR 
  RADIATION 
  — 
  O'BRIEN 
  111 
  

  

  mined 
  in 
  a 
  variety 
  of 
  ways. 
  The 
  most 
  fundamental 
  method 
  consists 
  

   in 
  converting 
  the 
  radiation 
  into 
  heat 
  by 
  absorption 
  in 
  a 
  blackened 
  

   surface, 
  and 
  measuring 
  the 
  amount 
  of 
  heat 
  produced 
  in 
  each 
  narrow 
  

   region 
  of 
  the 
  spectrum 
  by 
  the 
  rise 
  of 
  temperature 
  of 
  a 
  delicate 
  elec- 
  

   trical 
  thermometer. 
  This 
  is 
  analogous 
  to 
  quantitative 
  analysis 
  by 
  the 
  

   chemist. 
  The 
  energy 
  in 
  each 
  wave 
  length 
  having 
  been 
  determined, 
  

   it 
  may 
  be 
  plotted 
  as 
  a 
  graph 
  of 
  energy 
  against 
  wave 
  length, 
  such 
  a 
  

   plot 
  representing 
  the 
  spectral-energy 
  distribution 
  of 
  the 
  light 
  from 
  

   the 
  particular 
  source 
  measured. 
  

  

  Fortunately, 
  this 
  has 
  been 
  carried 
  out 
  for 
  solar 
  radiation 
  very 
  

   completely 
  and 
  with 
  high 
  accuracy 
  by 
  Dr. 
  Abbot 
  and 
  his 
  associates 
  at 
  

   the 
  Smithsonian 
  Astrophysical 
  Observatory, 
  so 
  that 
  the 
  spectral- 
  

   energy 
  distribution 
  of 
  sunlight 
  is 
  now 
  well 
  known. 
  Although 
  the 
  

   measurements 
  are 
  made 
  at 
  the 
  earth's 
  surface 
  and 
  are 
  thus 
  measure- 
  

   ments 
  of 
  sunlight 
  after 
  loss 
  by 
  transmission 
  through 
  the 
  earth's 
  atmos- 
  

   phere, 
  there 
  is 
  a 
  perfectly 
  definite 
  procedure 
  by 
  which 
  the 
  amount 
  of 
  

   this 
  loss 
  is 
  determined, 
  and 
  so 
  the 
  spectral 
  energy 
  outside 
  the 
  earth's 
  

   atmosphere 
  as 
  well 
  as 
  that 
  at 
  the 
  earth's 
  surface 
  is 
  known. 
  These 
  

   are 
  plotted 
  in 
  figure 
  1. 
  It 
  will 
  be 
  seen 
  that 
  in 
  sunlight 
  the 
  greatest 
  

   energy 
  occurs 
  in 
  the 
  visible 
  region 
  of 
  the 
  spectrum, 
  the 
  intensity 
  falling 
  

   off 
  rapidly 
  as 
  one 
  passes 
  beyond 
  the 
  violet 
  into 
  the 
  ultraviolet 
  region, 
  

   and 
  falling 
  off 
  more 
  slowly 
  as 
  one 
  goes 
  beyond 
  the 
  red 
  into 
  the 
  infra- 
  

   red. 
  

  

  In 
  order 
  to 
  act 
  upon 
  living 
  matter, 
  solar 
  radiation 
  must 
  be 
  absorbed 
  

   by 
  some 
  part 
  of 
  the 
  living 
  organism. 
  In 
  the 
  case 
  of 
  micro-organisms, 
  

   this 
  absorption 
  occurs 
  throughout 
  the 
  volume 
  of 
  the 
  whole 
  animal 
  or 
  

   plant, 
  much 
  of 
  the 
  radiation 
  passing 
  entirely 
  through 
  the 
  organism. 
  

   In 
  the 
  higher 
  animals 
  and 
  man, 
  practically 
  all 
  the 
  radiation 
  is 
  ab- 
  

   sorbed 
  in 
  the 
  skin, 
  no 
  significant 
  amount 
  penetrating 
  to 
  an 
  appreciable 
  

   depth. 
  In 
  animals, 
  even 
  the 
  skin 
  is 
  protected 
  by 
  the 
  hair, 
  which 
  thus 
  

   becomes 
  the 
  principal 
  absorber 
  of 
  light. 
  Oddly 
  enough, 
  this 
  absorp- 
  

   tion 
  by 
  hair 
  is 
  utilized 
  by 
  animals 
  in 
  at 
  least 
  one 
  important 
  vitamin 
  

   reaction. 
  When 
  radiation 
  of 
  wave 
  length 
  in 
  or 
  near 
  the 
  visible 
  spec- 
  

   trum 
  is 
  absorbed 
  by 
  living 
  matter, 
  the 
  energy 
  is 
  either 
  converted 
  into 
  

   heat 
  or 
  enters 
  directly 
  into 
  a 
  photochemical 
  reaction. 
  Either 
  or 
  both 
  

   of 
  these 
  effects 
  constitute 
  the 
  first 
  step 
  in 
  the 
  direct 
  action 
  of 
  sunlight 
  

   upon 
  a 
  living 
  organism. 
  Since 
  this 
  energy 
  transformation 
  must 
  oc- 
  

   cur 
  when 
  and 
  where 
  the 
  light 
  is 
  absorbed, 
  we 
  may 
  expect 
  a 
  primary 
  

   action 
  anywhere 
  within 
  a 
  small 
  organism. 
  In 
  larger 
  organisms, 
  and 
  

   in 
  particular 
  in 
  higher 
  animals 
  and 
  man, 
  the 
  primary 
  action 
  must 
  oc- 
  

   cur 
  at 
  the 
  surface 
  ; 
  that 
  is, 
  in 
  the 
  skin. 
  

  

  Human 
  skin 
  is 
  somewhat 
  different 
  from 
  that 
  of 
  any 
  animal, 
  al- 
  

   though 
  the 
  gross 
  structures 
  are 
  roughly 
  comparable. 
  Even 
  unpig- 
  

   mented 
  human 
  skin 
  absorbs 
  ultraviolet 
  light 
  strongly, 
  while 
  at 
  the 
  

  

  