﻿Chemistry 
  and 
  Physics. 
  215 
  

  

  between 
  the 
  lamp 
  and 
  the 
  objective. 
  By 
  rotating 
  this 
  wheel 
  so 
  

   as 
  to 
  interpose 
  the 
  proper 
  ray 
  -filter 
  in 
  the 
  path 
  of 
  the 
  beam 
  of 
  

   light, 
  it 
  was 
  possible 
  to 
  flash 
  signals 
  either 
  with 
  white, 
  or 
  with 
  

   infra-red, 
  or 
  with 
  ultra-violet 
  light. 
  To 
  use 
  the 
  telescope 
  it 
  was 
  

   only 
  necessary 
  to 
  point 
  the 
  tube 
  in 
  such 
  a 
  direction 
  as 
  to 
  cause 
  

   the 
  image 
  of 
  the 
  lamp 
  filament 
  to 
  be 
  accurately 
  superposed 
  upon 
  

   the 
  inverted 
  image 
  of 
  the 
  receiving 
  station. 
  As 
  regards 
  effi- 
  

   ciency, 
  the 
  following 
  comparative 
  data 
  may 
  be 
  mentioned. 
  A 
  

   parabolic 
  reflecting 
  lamp 
  of 
  35 
  cm. 
  aperture 
  carries 
  less 
  than 
  

   10 
  km. 
  The 
  flash-telescope, 
  when 
  used 
  in 
  broad 
  daylight, 
  car- 
  

   ries 
  30 
  km., 
  while 
  using 
  less 
  than 
  one-half 
  the 
  power 
  consumed 
  

   by 
  the 
  search 
  light. 
  

  

  Some 
  additional 
  advantages 
  of 
  Wood's 
  telescope 
  are: 
  (a) 
  the 
  

   beam 
  of 
  light 
  is 
  a 
  very 
  narrow 
  cone 
  (cross-section 
  about 
  6 
  ft. 
  

   at 
  a 
  distance 
  of 
  1 
  mile) 
  so 
  that 
  secrecy 
  is 
  assured, 
  (b) 
  the 
  tele- 
  

   scope 
  can 
  be 
  aimed 
  with 
  great 
  precision, 
  (c) 
  the 
  instrument 
  can 
  

   be 
  used 
  as 
  a 
  receiver 
  of 
  signals 
  sent 
  out 
  by 
  a 
  very 
  distant 
  station, 
  

   and 
  (d) 
  ordinary 
  displacements 
  of 
  the 
  lamp 
  with 
  respect 
  to 
  the 
  

   tube, 
  arising 
  from 
  shocks 
  incident 
  upon 
  transportation, 
  have 
  no 
  

   deleterious 
  influence 
  on 
  the 
  accuracy 
  of 
  pointing. 
  Case 
  (d) 
  

   does 
  not 
  hold 
  true 
  for 
  parabolic 
  reflecting 
  lamps. 
  

  

  "When 
  conditions 
  were 
  such 
  as 
  to 
  preclude 
  the 
  use 
  of 
  white 
  

   signals 
  in 
  full 
  daylight, 
  the 
  difficulty 
  was 
  overcome 
  by 
  employ- 
  

   ing 
  radiations 
  between 
  A6900 
  and 
  A7500. 
  The 
  ray 
  filters 
  were 
  

   made, 
  in 
  the 
  well-known 
  way, 
  by 
  staining 
  gelatin 
  with 
  appro- 
  

   priate 
  aniline 
  dyes. 
  In 
  general, 
  the 
  filter 
  was 
  compound, 
  one 
  

   gelatin 
  sheet 
  being 
  stained 
  with 
  cyanin 
  and 
  the 
  superposed 
  plate 
  

   with 
  almost 
  any 
  deep 
  orange 
  coloring 
  matter. 
  The 
  flash-tele- 
  

   scope 
  was 
  always 
  provided 
  with 
  two 
  infra-red 
  screens, 
  the 
  

   brighter 
  and 
  darker 
  filters 
  being 
  best 
  suited 
  for 
  signalling 
  over 
  

   distances 
  from 
  two 
  to 
  six 
  miles, 
  and 
  under 
  two 
  miles, 
  respec- 
  

   tively. 
  By 
  using 
  spectacles 
  or 
  field 
  glasses, 
  etc., 
  in 
  conjunction 
  

   with 
  color 
  screens 
  of 
  the 
  same 
  kind 
  as 
  in 
  the 
  sending 
  apparatus, 
  

   the 
  dazzling 
  daylight 
  was 
  decreased 
  in 
  intensity 
  to 
  a 
  very 
  much 
  

   greater 
  degree 
  than 
  the 
  infra-red 
  radiations, 
  thus 
  causing 
  the 
  

   distant 
  signal 
  lamp 
  to 
  assume 
  the 
  appearance 
  of 
  a 
  brilliant 
  star 
  

   on 
  an 
  approximately 
  black 
  landscape. 
  

  

  Secret 
  signalling 
  at 
  night 
  was 
  accomplished 
  by 
  using 
  ultra- 
  

   violet 
  rays. 
  The 
  filters 
  consisted 
  of 
  circular 
  discs 
  of 
  black 
  glass 
  

   having 
  an 
  oxide 
  of 
  nickel 
  as 
  the 
  coloring 
  base. 
  The 
  receiving 
  

   apparatus 
  was 
  essentially 
  a 
  short 
  focus 
  telescope, 
  of 
  very 
  wide 
  

   aperture, 
  provided 
  with 
  a 
  screen 
  coated 
  with 
  barium 
  platino- 
  

   cyanide 
  and 
  coinciding 
  with 
  a 
  focal 
  plane 
  of 
  the 
  condensing 
  

   objective. 
  This 
  screen 
  was 
  viewed 
  through 
  a 
  small 
  ocular. 
  The 
  

   ultra-violet 
  light 
  was 
  not 
  transmitted 
  by 
  the 
  screen, 
  of 
  course, 
  

   but 
  the 
  focal 
  spot 
  was 
  made 
  visible 
  by 
  the 
  bright 
  green 
  phos- 
  

   phorescence 
  of 
  the 
  barium 
  compound. 
  With 
  the 
  less 
  dense 
  fil- 
  

   ters 
  signals 
  were 
  received 
  at 
  a 
  distance 
  of 
  5-5 
  miles, 
  and 
  with 
  

   the 
  deeper 
  filters 
  at 
  2-5 
  miles. 
  

  

  