﻿C. 
  B. 
  Thwing- 
  Color 
  Photography. 
  389 
  

  

  ordinary 
  negative. 
  The 
  peculiarities 
  of 
  Lippmann's 
  method 
  

   consist, 
  first, 
  in 
  the 
  use 
  of 
  a 
  plate 
  which 
  is 
  transparent 
  and 
  free 
  

   from 
  grains 
  ; 
  second, 
  in 
  the 
  exposure 
  of 
  the 
  plate 
  with 
  its 
  film 
  

   side 
  resting 
  against 
  a 
  reflecting 
  surface 
  of 
  mercury. 
  The 
  inter- 
  

   ference 
  of 
  the 
  reflected 
  with 
  the 
  incident 
  ray 
  of 
  light 
  divides 
  

   the 
  film 
  into 
  a 
  number 
  of 
  layers 
  at 
  the 
  maxima 
  which 
  will 
  cor- 
  

   respond 
  in 
  their 
  distance 
  apart 
  with 
  the 
  wave 
  length 
  of 
  the 
  

   incident 
  light, 
  and 
  will, 
  therefore, 
  be 
  able 
  to 
  reproduce 
  by 
  

   reflection 
  the 
  color 
  which 
  produced 
  the 
  layers. 
  

  

  Lippmann 
  says 
  that 
  the 
  plates 
  are 
  positive 
  for 
  reflected 
  and 
  

   negative 
  for 
  transmitted 
  light 
  (negatif 
  par 
  transparence). 
  By 
  

   negative 
  he 
  means 
  showing 
  the 
  complementary 
  color. 
  In 
  that 
  

   one 
  of 
  Lippmann's 
  negatives 
  which 
  I 
  have 
  seen 
  and 
  all 
  those 
  

   obtained 
  by 
  myself, 
  the 
  plates 
  are 
  opaque 
  to 
  transmitted 
  light, 
  

   showing 
  only 
  differences 
  of 
  density 
  like 
  an 
  ordinary 
  negative. 
  

   The 
  reverse 
  side 
  of 
  the 
  plate, 
  however, 
  shows 
  the 
  comple- 
  

   mentary 
  colors, 
  somewhat 
  fainter 
  than 
  the 
  original 
  colors 
  which 
  

   appear 
  on 
  the 
  film 
  side 
  of 
  the 
  plate. 
  

  

  If 
  the 
  plates 
  were, 
  in 
  reality, 
  negative 
  by 
  transmitted 
  light, 
  

   it 
  might 
  be 
  possible 
  to 
  obtain 
  by 
  two 
  steps 
  instead 
  of 
  the 
  one 
  

   employed 
  in 
  ordinary 
  photography, 
  a 
  number 
  of 
  copies 
  from 
  a 
  

   single 
  negative. 
  The 
  remaining 
  method 
  is 
  to 
  copy 
  the 
  reflected 
  

   image, 
  and, 
  as 
  the 
  reflected 
  colors 
  are 
  bright, 
  this 
  may 
  not 
  

   prove 
  impossible. 
  In 
  my 
  experiments 
  certain 
  modifications 
  were 
  

   introduced 
  with 
  a 
  view 
  of 
  determining 
  several 
  points 
  which 
  

   are 
  not 
  brought 
  out 
  by 
  the 
  original 
  experiment 
  as 
  reported 
  by 
  

   Lippmann. 
  It 
  is 
  difficult 
  to 
  obtain 
  a 
  plate 
  which 
  shall 
  be 
  

   transparent 
  and 
  yet 
  possess 
  any 
  sufficient 
  degree 
  of 
  sensitive- 
  

   ness. 
  The 
  plates 
  I 
  have 
  found 
  most 
  satisfactory 
  hitherto 
  are 
  

   of 
  collodion 
  on 
  a 
  thin 
  substratum 
  of 
  albumen. 
  

  

  Following 
  is 
  the 
  formula 
  employed 
  : 
  

  

  i 
  Cadmium 
  bromide 
  25 
  g. 
  

  

  (1) 
  ^Alcohol 
  280 
  c.c. 
  

  

  ( 
  Hydrochloric 
  acid 
  5 
  c.c. 
  

  

  Of 
  (1) 
  5 
  c.c. 
  

  

  Ether 
  _■ 
  40 
  c.c. 
  

  

  Pyroxyline 
  2 
  g. 
  

  

  Sensitize 
  by 
  adding, 
  drop 
  by 
  drop, 
  a 
  solution 
  of 
  silver 
  nitrate, 
  

   1 
  g., 
  in 
  alcohol, 
  10 
  c.c, 
  and 
  pour 
  without 
  waiting 
  for 
  the 
  

   emulsion 
  to 
  ripen. 
  

  

  The 
  film 
  obtained 
  is 
  a 
  pale 
  opalescent 
  blue, 
  almost 
  perfectly 
  

   transparent, 
  and 
  requires 
  an 
  exposure 
  of 
  twenty 
  minutes 
  or 
  

   more 
  in 
  direct 
  sunlight 
  to 
  produce 
  images 
  of 
  the 
  green 
  and 
  

   red. 
  It 
  should 
  be 
  remarked, 
  however, 
  that 
  the 
  image 
  is 
  not 
  

   latent 
  but 
  appears 
  nearly 
  as 
  strong 
  without 
  the 
  use 
  of 
  a 
  devel- 
  

  

  