﻿112 
  ANNUAL 
  REPORT 
  SMITHSONIAN 
  INSTITUTION, 
  1943 
  

  

  red 
  end 
  of 
  the 
  spectrum, 
  unpigmented 
  skin 
  is 
  a 
  relatively 
  poor 
  ab- 
  

   sorber, 
  reflecting 
  back 
  much 
  of 
  the 
  light 
  energy 
  received. 
  This 
  is 
  

   shown 
  by 
  two 
  photographs 
  of 
  unpigmented 
  human 
  skin 
  which 
  are 
  

   reproduced 
  in 
  plate 
  1. 
  The 
  first 
  photograph 
  was 
  made 
  with 
  ultra- 
  

   violet 
  radiation, 
  while 
  the 
  second 
  was 
  made 
  with 
  infrared 
  radiation 
  at 
  

   a 
  wave 
  length 
  just 
  beyond 
  the 
  visible 
  spectrum. 
  For 
  comparison 
  a 
  

   block 
  of 
  white 
  magnesium 
  carbonate 
  was 
  held 
  in 
  the 
  hand 
  in 
  both 
  

   photographs. 
  This 
  was 
  a 
  good 
  diffuse 
  reflector 
  (i. 
  e., 
  "white") 
  in 
  the 
  

   ultraviolet 
  and 
  infrared 
  as 
  well 
  as 
  in 
  the 
  visible 
  spectrum. 
  

  

  Although 
  unpigmented 
  skin 
  reflects 
  much 
  of 
  the 
  visible 
  light 
  near 
  

   the 
  reel 
  end 
  of 
  the 
  spectrum 
  as 
  well 
  as 
  the 
  infrared 
  just 
  beyond, 
  that 
  

   which 
  is 
  absorbed 
  appears 
  to 
  produce 
  a 
  rather 
  specific 
  thermal 
  reac- 
  

   tion. 
  If 
  one 
  examines 
  a 
  cross 
  section 
  of 
  human 
  skin, 
  it 
  will 
  be 
  seen 
  

   that 
  the 
  overlying 
  layers 
  contain 
  no 
  pigment 
  materials 
  that 
  absorb 
  

   red 
  or 
  near-infrared 
  radiation 
  strongly. 
  Reflection 
  does 
  take 
  place 
  in 
  

   these 
  regions, 
  owing 
  to 
  the 
  many 
  discontinuities 
  in 
  refractive 
  index 
  

   produced 
  by 
  the 
  cell 
  boundaries 
  in 
  the 
  epidermis, 
  but 
  no 
  measurable 
  

   absorption 
  occurs 
  until 
  the 
  level 
  of 
  the 
  capillaries 
  is 
  reached 
  at 
  1 
  milli- 
  

   meter 
  or 
  so 
  beneath 
  the 
  surface. 
  Here 
  the 
  blood 
  pigments 
  absorb 
  

   strongly, 
  with 
  the 
  result 
  that 
  the 
  temperature 
  of 
  this 
  layer 
  is 
  elevated 
  

   above 
  the 
  surface 
  temperature 
  of 
  the 
  skin, 
  and 
  even 
  above 
  the 
  depth 
  

   temperature 
  of 
  the 
  body 
  when 
  the 
  skin 
  is 
  exposed 
  in 
  not-too-cool 
  air 
  to 
  

   intense 
  radiation. 
  This 
  effect 
  was 
  first 
  observed 
  by 
  Carl 
  Sonne, 
  who 
  

   measured 
  the 
  temperature 
  at 
  successive 
  depths 
  beneath 
  the 
  skin's 
  sur- 
  

   face 
  with 
  a 
  delicate 
  needle 
  thermocouple 
  and 
  found 
  a 
  marked 
  rise 
  in 
  

   temperature 
  under 
  intense 
  illumination. 
  The 
  conversion 
  of 
  radiation 
  

   to 
  heat 
  at 
  this 
  level 
  in 
  the 
  skin 
  raises 
  the 
  temperature 
  of 
  the 
  capillaries 
  

   above 
  adjacent 
  layers, 
  heat 
  being 
  conducted 
  both 
  to 
  the 
  cooler 
  skin 
  

   surface 
  and 
  the 
  cooler 
  tissues 
  at 
  a 
  depth. 
  Since 
  the 
  more 
  sensitive 
  

   innervation 
  is 
  above 
  the 
  capillary 
  layer, 
  it 
  should 
  be 
  possible 
  to 
  pro- 
  

   duce 
  without 
  discomfort 
  a 
  higher 
  temperature 
  in 
  the 
  capillary 
  blood 
  

   by 
  the 
  direct 
  absorption 
  of 
  radiation 
  in 
  it 
  than 
  by 
  conduction 
  of 
  heat 
  

   from 
  the 
  skin 
  surface 
  inward, 
  as 
  would 
  occur 
  with 
  a 
  hot 
  object 
  held 
  

   against 
  the 
  skin. 
  In 
  the 
  latter 
  case, 
  with 
  the 
  temperature 
  gradient 
  in- 
  

   ward, 
  the 
  nerve 
  endings 
  would 
  be 
  at 
  a 
  higher 
  temperature 
  than 
  the 
  

   capillaries. 
  Sonne 
  reports 
  capillary 
  temperatures 
  produced 
  by 
  light 
  

   absorption 
  comparable 
  to 
  high 
  fever 
  temperatures, 
  yet 
  without 
  dis- 
  

   comfort 
  to 
  the 
  patient, 
  and 
  without 
  a 
  corresponding 
  increase 
  of 
  either 
  

   surface 
  or 
  depth 
  temperature. 
  The 
  significance 
  of 
  this 
  effect 
  is 
  not 
  

   yet 
  determined. 
  It 
  must 
  occur 
  in 
  only 
  moderate 
  degree 
  when 
  human 
  

   skin 
  is 
  exposed 
  to 
  sunlight, 
  since 
  several 
  times 
  the 
  intensity 
  of 
  sunlight 
  

   may 
  be 
  borne 
  without 
  discomfort. 
  

  

  With 
  the 
  exception 
  of 
  this 
  thermal 
  effect 
  in 
  the 
  red 
  and 
  the 
  near- 
  

   infrared, 
  and 
  excepting 
  also 
  the 
  action 
  of 
  visible 
  light 
  upon 
  the 
  eye, 
  

  

  