﻿118 
  ANNUAL 
  REPORT 
  SMITHSONIAN 
  INSTITUTION, 
  1943 
  

  

  fulfilled, 
  but 
  many 
  other 
  light-absorbing 
  substances 
  are 
  present. 
  It 
  

   is 
  not 
  surprising, 
  therefore, 
  that 
  the 
  spectral 
  response 
  for 
  the 
  anti- 
  

   rachitic 
  effect 
  of 
  ultraviolet 
  radiation 
  directly 
  upon 
  the 
  animal's 
  skin 
  

   should 
  be 
  modified 
  somewhat 
  from 
  the 
  response 
  of 
  the 
  pure 
  sterol. 
  

   Knudson 
  and 
  Benford 
  have 
  measured 
  this 
  response 
  in 
  albino 
  rats, 
  

   their 
  results 
  being 
  shown 
  also 
  in 
  figure 
  5. 
  The 
  response 
  in 
  human 
  

   skin 
  may 
  not 
  be 
  identical 
  with 
  that 
  occurring 
  in 
  the 
  albino 
  rat, 
  but 
  

   may 
  well 
  be 
  very 
  similar, 
  so 
  the 
  results 
  of 
  Knudson 
  and 
  Benford 
  pre- 
  

   sent 
  the 
  best 
  approximation 
  to 
  date. 
  

  

  In 
  order 
  that 
  we 
  may 
  calculate 
  the 
  antirachitic 
  effect 
  of 
  sunlight 
  2 
  

   under 
  a 
  variety 
  of 
  conditions 
  when 
  acting 
  directly 
  upon 
  animal 
  skin, 
  

   it 
  is 
  necessary 
  that 
  we 
  know 
  more 
  than 
  the 
  spectral 
  response 
  per 
  unit 
  

   energy 
  shown 
  in 
  figure 
  5. 
  In 
  addition, 
  it 
  is 
  necessary 
  to 
  have 
  the 
  

   spectral-energy 
  distribution 
  at 
  the 
  short-wave-length 
  end 
  of 
  the 
  sun's 
  

   spectrum 
  for 
  the 
  several 
  conditions 
  under 
  which 
  we 
  wish 
  to 
  calculate 
  

   the 
  effect. 
  Because 
  of 
  the 
  strong 
  selective 
  absorption 
  by 
  ozone 
  in 
  

   this 
  region 
  of 
  the 
  spectrum, 
  the 
  energy 
  is 
  dropping 
  rapidly 
  as 
  one 
  

   proceeds 
  to 
  shorter 
  wave 
  lengths. 
  For 
  this 
  reason 
  the 
  usual 
  thermal 
  

   method 
  for 
  measuring 
  spectral-energy 
  distribution 
  is 
  not 
  as 
  satis- 
  

   factory 
  as 
  are 
  certain 
  photographic 
  procedures. 
  In 
  these, 
  suitable 
  

   precautions 
  must 
  be 
  taken 
  as 
  have 
  been 
  described 
  elsewhere, 
  and 
  a 
  

   double 
  dispersion 
  spectrographs 
  instrument 
  must 
  be 
  used 
  to 
  eliminate 
  

   the 
  effects 
  of 
  scattered 
  light. 
  The 
  details 
  of 
  these 
  measurements 
  will 
  

   be 
  published 
  elsewhere. 
  The 
  results 
  are 
  shown 
  in 
  figure 
  6 
  for 
  the 
  

   spectral 
  intensity 
  of 
  solar 
  radiation 
  as 
  received 
  at 
  the 
  earth's 
  surface 
  

   at 
  sea 
  level 
  through 
  a 
  clear 
  zenith 
  atmosphere 
  (air 
  mass 
  1) 
  for 
  two 
  

   ozone 
  quantities. 
  The 
  upper 
  curve 
  is 
  for 
  total 
  ozone 
  in 
  a 
  zenith 
  

   atmosphere 
  equal 
  to 
  2.0 
  millimeters 
  of 
  the 
  pure 
  gas 
  at 
  standard 
  tempera- 
  

   ture 
  and 
  pressure, 
  while 
  the 
  lower 
  curve 
  is 
  for 
  2.8 
  millimeters 
  of 
  ozone 
  

   under 
  the 
  same 
  condition. 
  These 
  curves 
  have 
  been 
  smoothed 
  to 
  

   eliminate 
  the 
  Fraunhofer 
  structure 
  while 
  still 
  preserving 
  the 
  correct 
  

   average 
  ordinates 
  over 
  any 
  small 
  wave-length 
  interval. 
  This 
  is 
  to 
  

   simplify 
  the 
  graphical 
  integration 
  to 
  be 
  carried 
  out 
  as 
  described 
  below. 
  

  

  In 
  general, 
  sunlight 
  must 
  reach 
  the 
  earth's 
  surface 
  after 
  passing 
  

   obliquely 
  through 
  the 
  earth's 
  atmosphere 
  at 
  some 
  angle 
  Z 
  with 
  the 
  

   zenith. 
  This 
  results 
  in 
  an 
  increase 
  in 
  path 
  through 
  each 
  stratum 
  of 
  

   the 
  atmosphere 
  in 
  the 
  ratio 
  of 
  the 
  secant 
  of 
  the 
  zenith 
  angle. 
  The 
  

   resultant 
  increase 
  in 
  absorption 
  by 
  the 
  ozone 
  and 
  increase 
  in 
  scattering 
  

   by 
  the 
  air 
  molecules 
  can 
  be 
  calculated. 
  The 
  result 
  is 
  shown 
  in 
  the 
  

   curves 
  of 
  figures 
  7 
  and 
  8 
  for 
  a 
  series 
  of 
  air 
  masses 
  (i. 
  e., 
  secants 
  of 
  the 
  

   zenith 
  angle), 
  and 
  for 
  two 
  quantities 
  of 
  ozone, 
  2.0 
  and 
  2.8 
  millimeters 
  

   S. 
  T. 
  P. 
  in 
  a 
  zenith 
  atmosphere. 
  

  

  a 
  The 
  writer 
  wishes 
  to 
  acknowledge 
  the 
  assistance 
  of 
  Mrs. 
  F. 
  Dana 
  Miller 
  in 
  making 
  

   calculations 
  or 
  antirachitic 
  effects 
  under 
  a 
  research 
  grant 
  from 
  the 
  Wisconsin 
  Alumni 
  Re- 
  

   search 
  Foundation. 
  

  

  