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  ANNUAL 
  EEPORT 
  SMITHSONIAN 
  INSTITUTION, 
  1927 
  

  

  With 
  this 
  exact 
  definition 
  of 
  what 
  had 
  formerly 
  been 
  a 
  rough 
  

   and 
  ready 
  classification, 
  several 
  observers 
  have 
  attained 
  great 
  distinc- 
  

   tion 
  by 
  their 
  long-continned 
  researches 
  in 
  stellar 
  photometry, 
  which 
  

   means 
  the 
  assignment 
  of 
  exact 
  magnitudes 
  to 
  the 
  stars. 
  Some 
  of 
  

   these 
  investigators 
  made 
  visual 
  observations 
  at 
  the 
  telescope, 
  but, 
  as 
  

   in 
  other 
  lines 
  in 
  astronomy, 
  photography 
  was 
  employed 
  by 
  others 
  

   with 
  great 
  advantage. 
  The 
  ordinary 
  photographic 
  plate 
  is 
  most 
  

   sensitive 
  to 
  blue 
  and 
  violet 
  light, 
  while 
  the 
  eye 
  is 
  most 
  sensitive 
  to 
  

   light-green 
  and 
  yellow. 
  Hence, 
  while 
  some 
  observers 
  were 
  contented 
  

   to 
  measure 
  photographic 
  brightness 
  by 
  ordinary 
  plates, 
  others 
  inter- 
  

   posed 
  absorbing 
  screens 
  suitable 
  to 
  restrict 
  the 
  rays 
  to 
  the 
  most 
  effec- 
  

   tiA'e 
  visible 
  region, 
  and 
  observed 
  on 
  orthochromatic 
  plates. 
  

  

  Thus 
  the 
  values 
  of 
  brightness 
  of 
  the 
  stars 
  have 
  been 
  measured 
  not 
  

   only 
  photographically 
  but 
  photo 
  visually, 
  as 
  we 
  may 
  say, 
  and 
  the 
  

   magnitudes 
  have 
  been 
  recorded 
  on 
  both 
  scales. 
  Reddish 
  stars 
  are 
  

   naturally 
  much 
  fainter 
  according 
  to 
  photographic 
  than 
  according 
  to 
  

   visual 
  or 
  photovisual 
  magnitudes. 
  The 
  difference 
  of 
  the 
  two 
  scales 
  

   gives, 
  indeed, 
  a 
  measure 
  of 
  the 
  effective 
  colors 
  of 
  stars, 
  which, 
  in 
  cases 
  

   of 
  objects 
  too 
  faint 
  for 
  spectroscopic 
  analysis, 
  is 
  valuable 
  as 
  an 
  index 
  

   of 
  the 
  character 
  of 
  the 
  spectrum. 
  

  

  By 
  persevering 
  work 
  along 
  these 
  lines, 
  there 
  have 
  been 
  established 
  

   in 
  recent 
  times 
  standard 
  series 
  of 
  stars 
  of 
  fainter 
  and 
  fainter 
  ap- 
  

   parent 
  luminosity, 
  so 
  that 
  the 
  observer 
  in 
  any 
  part 
  of 
  the 
  heavens, 
  

   by 
  making 
  comparison 
  photographs 
  from 
  time 
  to 
  time 
  v/ithin 
  stand- 
  

   ardized 
  stellar 
  regions, 
  may 
  reduce 
  his 
  observed 
  magnitudes 
  to 
  a 
  

   well-settled 
  standard 
  scale, 
  no 
  matter 
  what 
  the 
  prevailing 
  sl<y 
  con- 
  

   ditions 
  .for 
  the 
  night 
  may 
  be. 
  

  

  With 
  these 
  advantages 
  it 
  has 
  become 
  possible 
  to 
  count 
  accurately 
  

   the 
  numbers 
  of 
  stars 
  brighter 
  than 
  specified 
  magnitudes, 
  as 
  photo- 
  

   graphed 
  Avithin 
  selected 
  areas, 
  each 
  of 
  small 
  but 
  definite 
  extent, 
  and 
  

   numerous 
  enough 
  to 
  give 
  a 
  fair 
  estimate 
  of 
  the 
  star 
  population 
  of 
  

   the 
  whole 
  heavens. 
  In 
  the 
  following 
  table, 
  prepared 
  by 
  Scares 
  and 
  

   van 
  Rhijn, 
  wo 
  see 
  how 
  the 
  numbers 
  run. 
  

  

  Table 
  I. 
  — 
  Numhers 
  of 
  stars 
  brighter 
  than 
  a 
  given 
  magnitude 
  

  

  