﻿EFFECTS 
  OF 
  SOLAR 
  RADIATION 
  — 
  O'BRIEN 
  117 
  

  

  chicken) 
  as 
  compared 
  to 
  tests 
  on 
  mammals 
  (usually 
  the 
  rat). 
  This 
  

   led 
  to 
  the 
  discovery 
  that 
  at 
  least 
  one 
  other 
  substance, 
  T-dehydro- 
  

   cholesterol, 
  could 
  be 
  converted 
  into 
  vitamin 
  D 
  by 
  exposure 
  to 
  light. 
  

   Ergosterol 
  is 
  a 
  characteristic 
  plant 
  sterol, 
  while 
  7-dehydro-cholesterol 
  

   is 
  a 
  sterol 
  found 
  in 
  animal 
  substances. 
  It 
  seems 
  probable 
  therefore 
  

   that 
  it 
  is 
  7-dehydro-cholesterol 
  and 
  not 
  ergosterol 
  which 
  is 
  acted 
  upon 
  

   when 
  human 
  skin 
  is 
  exposed 
  to 
  sunlight, 
  and 
  so 
  converted 
  into 
  vitamin 
  

   D, 
  which, 
  entering 
  the 
  peripheral 
  blood 
  stream, 
  prevents 
  or 
  cures 
  

   rickets 
  in 
  the 
  child 
  so 
  exposed. 
  It 
  was 
  first 
  shown 
  by 
  Kon, 
  Daniels, 
  

   and 
  Steenbock 
  that 
  the 
  quantum 
  efficiency 
  of 
  the 
  photochemical 
  reac- 
  

  

  ERGOSTEROL 
  

   EFFICIENCY 
  

  

  Z500 
  2600 
  2700 
  2800 
  2900 
  3000 
  3100 
  3200 
  

  

  WAVELENGTH 
  IN 
  ANGSTROMS 
  

  

  Figube 
  5. 
  — 
  Ultraviolet 
  absorption 
  of 
  provitamin 
  sterols 
  and 
  relative 
  antirachitic 
  

  

  reaction. 
  

  

  tion 
  converting 
  the 
  sterol 
  into 
  vitamin 
  D 
  is 
  substantially 
  independent 
  

   of 
  wave 
  length, 
  and 
  there 
  is 
  reason 
  to 
  expect 
  such 
  a 
  result. 
  This 
  

   would 
  mean 
  Jhat 
  the 
  energy 
  efficiency 
  of 
  the 
  reaction 
  could 
  be 
  arrived 
  

   at 
  from 
  the 
  characteristic 
  absorption 
  of 
  the 
  sterols 
  as 
  a 
  function 
  of 
  

   wave 
  length. 
  These 
  absorption 
  curves 
  are 
  shown 
  in 
  figure 
  5, 
  and 
  it 
  

   should 
  be 
  necessary 
  only 
  to 
  divide 
  the 
  ordinates 
  by 
  the 
  energy 
  value 
  

   of 
  the 
  quantum 
  at 
  each 
  wave 
  length 
  (i. 
  e., 
  multiply 
  the 
  ordinates 
  by 
  

   the 
  wave 
  lengths) 
  to 
  obtain 
  the 
  spectral-response 
  curve. 
  However, 
  

   such 
  a 
  spectral-response 
  curve 
  would 
  be 
  correct 
  only 
  when 
  the 
  sterol 
  

   was 
  irradiated 
  in 
  dilute 
  solution 
  and 
  in 
  the 
  absence 
  of 
  other 
  con- 
  

   taminating 
  substances 
  exhibiting 
  masking 
  absorptions 
  in 
  the 
  same 
  

   spectral 
  regions. 
  In 
  animal 
  skin 
  the 
  condition 
  of 
  dilution 
  is 
  no 
  doubt 
  

  

  566766^4 
  9 
  

  

  