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  ANNUAL 
  REPORT 
  SMITHSONIAN 
  INSTITUTION, 
  1922. 
  

  

  but 
  far 
  too 
  long 
  to 
  include 
  here; 
  for, 
  after 
  all, 
  lightning, 
  whatever 
  

   its 
  interest 
  and 
  importance, 
  is 
  only 
  a 
  cloud 
  incident. 
  

  

  When 
  the 
  storm 
  is 
  close 
  by 
  it 
  is 
  easy 
  to 
  see 
  that 
  lightning 
  consists 
  

   of 
  one 
  or 
  more 
  sinuous 
  (not 
  zigzag, 
  as 
  the 
  artist 
  paints 
  it) 
  lines 
  or 
  

   streaks 
  of 
  vivid 
  white 
  or 
  pink. 
  (PI. 
  25, 
  fig. 
  2.) 
  Often 
  there 
  is 
  one 
  

   main 
  trunk 
  with 
  a 
  number 
  of 
  branches, 
  all 
  occurring 
  at 
  the 
  same 
  

   instant, 
  while 
  at 
  other 
  times 
  there 
  are 
  two 
  or 
  more 
  simultaneous 
  

   disconnected 
  streaks. 
  Frequently 
  the 
  discharge, 
  instead 
  of 
  being 
  all 
  

   at 
  once, 
  continues 
  flickering 
  and, 
  on 
  rare 
  occasions, 
  even 
  stationary, 
  

   like 
  a 
  glowing 
  wire, 
  during 
  a 
  whole 
  second 
  or 
  more. 
  

  

  The 
  rainbow. 
  — 
  Unlike 
  the 
  phenomenon 
  of 
  lightning, 
  the 
  rainbow 
  

   is 
  all 
  beauty 
  and 
  splendor, 
  with 
  nothing 
  whatever 
  of 
  the 
  appalling 
  — 
  

   save 
  alone 
  to 
  the 
  physicist 
  who 
  undertakes 
  to 
  explain 
  fully 
  and 
  

   clearly 
  its 
  every 
  detail, 
  a 
  most 
  difficult 
  task. 
  Perhaps 
  this 
  assertion 
  

   may 
  seem 
  strange 
  when 
  so 
  many 
  textbooks, 
  even 
  some 
  that 
  are 
  

   quite 
  elementary, 
  profess 
  to 
  explain 
  the 
  rainbow 
  so 
  simply 
  that 
  a 
  

   child 
  can 
  understand 
  it. 
  But 
  in 
  this 
  particular 
  nearly 
  all 
  these 
  books 
  

   have 
  the 
  same 
  fault 
  — 
  they 
  "explain" 
  beautifully 
  that 
  which 
  does 
  

   not 
  occur 
  and 
  leave 
  unexplained 
  that 
  which 
  does. 
  

  

  The 
  ordinary 
  rainbow 
  seen 
  in 
  a 
  sheet 
  of 
  water 
  drops 
  — 
  rain, 
  spray, 
  

   fog 
  — 
  is 
  a 
  group 
  of 
  circular 
  or 
  nearly 
  circular 
  arcs 
  of 
  colors 
  whose 
  

   common 
  center 
  is 
  on 
  the 
  extension, 
  in 
  the 
  direction 
  of 
  the 
  observer's 
  

   shadow, 
  of 
  the 
  straight 
  line 
  connecting 
  his 
  eye 
  with 
  the 
  luminous 
  

   source, 
  which, 
  as 
  everyone 
  knows, 
  is 
  nearly 
  always 
  the 
  sun, 
  but 
  not 
  

   quite, 
  for 
  once 
  in 
  a 
  while 
  the 
  moon, 
  too, 
  produces 
  a 
  very 
  pretty 
  bow. 
  

  

  Frequently 
  there 
  are 
  two 
  entirely 
  different 
  bows. 
  The 
  inner 
  one, 
  

   known 
  as 
  the 
  'primary, 
  with 
  red 
  outer 
  border 
  of 
  about 
  42° 
  ra- 
  

   dius, 
  and 
  blue 
  to 
  violet 
  inner 
  border, 
  is 
  much 
  brighter 
  than 
  the 
  

   larger, 
  or 
  secondary 
  bow, 
  of 
  about 
  50° 
  inner 
  radius, 
  and 
  whose 
  colors 
  

   occur 
  in 
  the 
  reverse 
  order 
  of 
  those 
  of 
  the 
  primary 
  bow. 
  In 
  ad- 
  

   dition 
  to 
  these 
  two 
  conspicuous 
  bows 
  one 
  can 
  often 
  see 
  from 
  one 
  

   to, 
  rarely, 
  half 
  a 
  dozen 
  parallel, 
  colored 
  arcs 
  just 
  inside 
  the 
  pri- 
  

   mary 
  bow, 
  and 
  a 
  smaller 
  number 
  just 
  outside 
  the 
  secondary 
  bow. 
  

   These 
  are 
  known 
  as 
  the 
  supernumerary 
  bows. 
  Most 
  of 
  these 
  phe- 
  

   nomena 
  are 
  shown 
  in 
  plate 
  26, 
  figure 
  1, 
  copied, 
  by 
  kind 
  permission, 
  

   from 
  a 
  remarkably 
  fine 
  picture 
  taken 
  by 
  Mr. 
  G. 
  A. 
  Clarke, 
  of 
  

   Aberdeen, 
  Scotland. 
  

  

  A 
  careful 
  observer 
  will 
  soon 
  see 
  that 
  there 
  is 
  less 
  light 
  (that 
  

   the 
  clouds 
  look 
  darker) 
  between 
  the 
  two 
  bows 
  than 
  outside 
  the 
  

   secondary 
  or 
  within 
  the 
  primary 
  ; 
  that 
  the 
  colors 
  seen 
  are 
  not 
  alwaj's 
  

   the 
  same 
  ; 
  that 
  the 
  band 
  of 
  am' 
  given 
  color 
  varies 
  in 
  angular 
  width, 
  

   even, 
  occasionally, 
  from 
  place 
  to 
  place 
  in 
  the 
  same 
  bow; 
  and 
  that 
  

   the 
  purity 
  of 
  the 
  colors 
  varies 
  from 
  time 
  to 
  time. 
  As 
  one 
  would 
  

   naturally 
  suspect, 
  all, 
  or 
  nearly 
  all, 
  these 
  differences 
  depend 
  on 
  the 
  

  

  