﻿110 
  

  

  ANNUAL 
  REPORT 
  SMITHSONIAN 
  INSTITUTION, 
  194 
  3 
  

  

  have 
  to 
  do 
  with 
  biological 
  effects, 
  direct 
  or 
  indirect, 
  of 
  sunlight 
  upon 
  

   the 
  animal 
  organism. 
  They 
  are 
  specific 
  and 
  depend 
  upon 
  the 
  wave 
  

   length 
  and 
  character 
  of 
  the 
  light. 
  For 
  this 
  reason 
  we 
  must 
  start 
  by 
  

   considering 
  something 
  of 
  the 
  nature 
  of 
  the 
  radiant 
  energy 
  which 
  we 
  

   receive 
  from 
  the 
  sun. 
  

  

  The 
  light 
  from 
  most 
  sources 
  is 
  made 
  up 
  of 
  many 
  different 
  frequencies 
  

   or 
  wave 
  lengths, 
  and 
  sunlight 
  is 
  no 
  exception 
  to 
  this 
  rule. 
  Such 
  light 
  

   may 
  be 
  analyzed 
  into 
  a 
  spectrum 
  with 
  a 
  prism 
  and 
  the 
  wave 
  lengths 
  

   separated 
  from 
  each 
  other, 
  much 
  as 
  a 
  chemist 
  might 
  make 
  a 
  qualitative 
  

   analysis 
  to 
  determine 
  the 
  elements 
  of 
  which 
  some 
  material 
  or 
  com- 
  

  

  WAVELENGTH 
  IN 
  MICRONS 
  

  

  Figube 
  1. 
  — 
  Distribution 
  of 
  energy 
  in 
  the 
  solar 
  spectrum 
  (Abbot) 
  with 
  new 
  exten- 
  

   sion 
  at 
  ultraviolet 
  end. 
  

  

  pound 
  is 
  composed. 
  If 
  light 
  from 
  a 
  narrow 
  source 
  or 
  slit 
  is 
  allowed 
  

   to 
  pass 
  through 
  a 
  prism, 
  a 
  series 
  of 
  images 
  will 
  be 
  produced 
  corre- 
  

   sponding 
  to 
  the 
  several 
  wave 
  lengths 
  present, 
  and 
  so 
  a 
  spectrum 
  is 
  

   formed. 
  This 
  spectrum 
  shows 
  the 
  familiar 
  sequence 
  of 
  colors 
  of 
  the 
  

   rainbow 
  from 
  violet 
  at 
  the 
  short-wave-length 
  end 
  through 
  blue, 
  green, 
  

   yellow, 
  orange, 
  to 
  red 
  at 
  the 
  longest 
  wave 
  length 
  which 
  can 
  be 
  seen. 
  

   There 
  are, 
  of 
  course, 
  wave 
  lengths 
  beyond 
  the 
  range 
  to 
  which 
  the 
  eye 
  

   is 
  sensitive. 
  At 
  longer 
  wave 
  length 
  than 
  the 
  red 
  is 
  the 
  infrared 
  

   extending 
  for 
  many 
  octaves, 
  and 
  at 
  shorter 
  wave 
  length 
  than 
  the 
  

   violet 
  is 
  the 
  ultraviolet, 
  a 
  region 
  which 
  will 
  be 
  of 
  particular 
  interest 
  

   to 
  us 
  here. 
  

  

  It 
  is 
  of 
  interest 
  to 
  know 
  what 
  wave 
  lengths 
  are 
  present, 
  but 
  it 
  is 
  

   even 
  more 
  important 
  to 
  know 
  how 
  much 
  of 
  each. 
  This 
  can 
  be 
  deter- 
  

  

  