﻿114 
  

  

  ANNUAL 
  REPORT 
  SMITHSONIAN 
  INSTITUTION, 
  194 
  3 
  

  

  the 
  subsequent 
  pigmentation 
  or 
  tanning 
  of 
  the 
  skin 
  has 
  been 
  a 
  dis- 
  

   puted 
  point. 
  The 
  pigment 
  of 
  tanned 
  skin 
  is 
  melanin, 
  which 
  appears 
  

   physiologically 
  inert, 
  produced 
  by 
  an 
  oxidation 
  reaction 
  following 
  ex- 
  

   posure 
  to 
  light. 
  It 
  seems 
  most 
  reasonable 
  to 
  suppose 
  that 
  the 
  pri- 
  

   mary 
  function 
  of 
  pigment 
  is 
  the 
  protection 
  it 
  affords 
  the 
  underlying- 
  

   portions 
  of 
  the 
  skin 
  and 
  blood 
  stream 
  from 
  further 
  excessive 
  exposure. 
  

   It 
  is 
  worth 
  while 
  to 
  examine 
  in 
  more 
  detail 
  the 
  ultraviolet 
  end 
  of 
  

   the 
  sun's 
  spectrum 
  responsible 
  for 
  this 
  reaction. 
  The 
  area 
  under 
  a 
  

   spectral-energy 
  distribution 
  curve 
  represents 
  the 
  energy 
  in 
  that 
  

   spectral 
  region. 
  It 
  will 
  be 
  evident 
  from 
  figure 
  1 
  that 
  the 
  area 
  under 
  

   the 
  solar-energy 
  distribution 
  curve 
  for 
  all 
  wave 
  lengths 
  shorter 
  than 
  

  

  3000 
  3100 
  3200 
  

  

  WAVELENGTH 
  IN 
  ANGSTROMS 
  

  

  Figtjbe 
  3. 
  — 
  Ultraviolet 
  absorption 
  of 
  ozone. 
  

  

  3150 
  angstroms 
  represents 
  less 
  than 
  one-thousandth 
  of 
  the 
  area 
  under 
  

   the 
  total 
  curve. 
  The 
  small 
  energy 
  involved 
  renders 
  the 
  effects 
  pro- 
  

   duced 
  by 
  this 
  region 
  of 
  the 
  spectrum 
  the 
  more 
  remarkable. 
  

  

  It 
  is 
  noteworthy 
  that 
  at 
  the 
  earth's 
  surface 
  the 
  sun's 
  spectrum 
  ter- 
  

   minates 
  very 
  abruptly 
  at 
  about 
  2900 
  angstrom 
  units. 
  The 
  spectrum 
  

   of 
  the 
  stars 
  and 
  all 
  known 
  heavenly 
  bodies 
  terminates 
  at 
  about 
  this 
  

   same 
  wave 
  length, 
  and 
  long 
  ago 
  the 
  conclusion 
  was 
  inevitable 
  that 
  

   something 
  in 
  the 
  earth's 
  atmosphere 
  must 
  be 
  absorbing 
  abruptly 
  at 
  

   this 
  point. 
  It 
  is 
  easy 
  to 
  produce 
  much 
  shorter 
  wavelengths 
  from 
  

   artificial 
  light 
  sources, 
  and 
  there 
  is 
  no 
  reason 
  to 
  attribute 
  this 
  abrupt 
  

   termination 
  to 
  anything 
  characteristic 
  of 
  the 
  emission 
  spectrum 
  of 
  

   the 
  sun 
  and 
  stars. 
  More 
  than 
  50 
  years 
  ago 
  Hartley 
  correctly 
  attri- 
  

   buted 
  this 
  abrupt 
  termination 
  to 
  absorption 
  by 
  the 
  gas 
  ozone, 
  tria- 
  

  

  