﻿and 
  1) 
  iff 
  ruction- 
  Telescopes. 
  510 
  

  

  be 
  poured 
  while 
  warm 
  over 
  the 
  silver 
  surface, 
  and 
  the 
  plate 
  

   set 
  on 
  edge 
  to 
  dry. 
  This 
  takes 
  place 
  in 
  five 
  or 
  ten 
  minutes, 
  

   and 
  the 
  surface 
  now 
  shows 
  interference 
  colours. 
  The 
  film 
  is 
  

   now 
  sensitized 
  by 
  a 
  ten-second 
  immersion 
  in 
  a 
  very 
  dilute 
  

   solution 
  of 
  bichromate 
  of 
  potash 
  (plate 
  dipped 
  in 
  it 
  should 
  be 
  

   coloured 
  a 
  very 
  pale 
  yellow 
  only) 
  and 
  dried 
  in 
  the 
  dark 
  *. 
  

  

  An 
  impression 
  is 
  taken 
  by 
  exposure 
  under 
  a 
  negative, 
  the 
  

   plate 
  is 
  held 
  for 
  a 
  moment 
  in 
  cold 
  water 
  and 
  then 
  washed 
  

   in 
  a 
  stream 
  of 
  fairly 
  hot 
  water. 
  This 
  is 
  the 
  fussy 
  part 
  of 
  

   the 
  operation 
  ; 
  for 
  if 
  the 
  stream 
  is 
  too 
  violent 
  or 
  the 
  water 
  

   too 
  hot 
  it 
  will 
  wash 
  away 
  everything, 
  and 
  if 
  it 
  is 
  too 
  feeble 
  

   some 
  gelatine 
  will 
  be 
  left 
  between 
  the 
  undissolved 
  zones. 
  I 
  

   find 
  that 
  pouring 
  from 
  a 
  beaker 
  held 
  at 
  a 
  height 
  of 
  10 
  centim. 
  

   gives 
  a 
  stream 
  of 
  the 
  proper 
  strength, 
  and 
  the 
  temperature 
  

   should 
  be 
  about 
  what 
  the 
  hand 
  can 
  bear 
  comfortably. 
  A 
  half 
  

   minute's 
  washing 
  is 
  enough, 
  and 
  the 
  plate 
  must 
  now 
  be 
  flooded 
  

   with 
  alcohol 
  and 
  then 
  with 
  ether, 
  to 
  prevent 
  the 
  slow 
  solution 
  

   of 
  gelatine 
  during 
  the 
  drying, 
  which 
  always 
  coats 
  the 
  clear 
  

   zones 
  with 
  a 
  thin 
  film. 
  The 
  plate 
  will 
  dry 
  in 
  a 
  few 
  seconds, 
  

   and 
  if 
  it 
  has 
  been 
  properly 
  prepared 
  the 
  odd 
  zones 
  will 
  be 
  

   clean 
  bright 
  silver, 
  and 
  the 
  even 
  ones 
  covered 
  with 
  a 
  thin 
  

   film 
  which 
  appears 
  white 
  by 
  reflected 
  light. 
  The 
  plate 
  is 
  

   now 
  covered 
  with 
  iodine 
  crystals, 
  which 
  convert 
  the 
  exposed 
  

   silver 
  into 
  iodide, 
  and 
  the 
  process 
  can 
  be 
  watched 
  by 
  holding 
  

   the 
  plate 
  above 
  the 
  head 
  and 
  looking 
  at 
  the 
  under 
  side. 
  The 
  

   iodide 
  zones 
  will 
  appear 
  white, 
  and 
  on 
  shaking 
  off 
  the 
  iodine 
  

   transparent 
  by 
  transmitted 
  light. 
  

  

  A 
  strong 
  solution 
  of 
  sodium 
  thiosulphate 
  is 
  now 
  applied, 
  

   which 
  dissolves 
  the 
  silver 
  iodide, 
  and 
  the 
  plate 
  is 
  then 
  rubbed 
  

   with 
  the 
  fingers 
  under 
  warm 
  water 
  until 
  the 
  gelatine 
  is 
  all 
  

   removed. 
  If 
  everything 
  goes 
  well 
  a 
  most 
  beautiful 
  plate 
  is 
  

   the 
  result. 
  The 
  silver 
  rings 
  are 
  as 
  sharp 
  as 
  if 
  cut 
  with 
  a 
  knife, 
  

   and 
  the 
  spaces 
  between 
  them 
  are 
  quite 
  clean, 
  even 
  where 
  

   there 
  are 
  fifty 
  zones 
  to 
  the 
  millimetre. 
  Such 
  a 
  plate 
  used 
  

   with 
  transmitted 
  light 
  is 
  much 
  better 
  than 
  the 
  usual 
  photo- 
  

   graphic 
  type, 
  in 
  that 
  the 
  clear 
  spaces 
  are 
  perfectly 
  clean 
  and 
  

   transparent, 
  and 
  the 
  dark 
  ones 
  perfectly 
  opaque, 
  which, 
  as 
  I 
  

   said 
  before, 
  is 
  usually 
  not 
  the 
  case. 
  

  

  It 
  is 
  not 
  necessary 
  to 
  make 
  the 
  zones 
  directly 
  on 
  the 
  prism, 
  

   though 
  I 
  tried 
  this 
  first. 
  It 
  is 
  essentially 
  the 
  same 
  thing 
  if 
  

   we 
  make 
  them 
  on 
  a 
  plate 
  of 
  glass 
  and 
  fasten 
  this 
  to 
  the 
  

   prism 
  face 
  with 
  Canada 
  balsam, 
  the 
  silver 
  side 
  out 
  of 
  course. 
  

   The 
  brilliancy 
  of 
  the 
  image 
  produced 
  by 
  this 
  arrangement 
  is 
  

   very 
  great, 
  but 
  since 
  the 
  reflecting 
  surface 
  is 
  at 
  an 
  angle 
  of 
  

  

  * 
  Lately 
  I 
  have 
  added 
  the 
  bichromate 
  to 
  the 
  gelatine 
  before 
  flowing- 
  

   the 
  plate-, 
  and 
  find 
  the 
  process 
  quicker 
  and 
  the 
  results 
  better. 
  . 
  . 
  

  

  - 
  2N2 
  

  

  