234        Photographic  Development  by  Gas  Light.    { AmMa" mhifrm* 
PHOTOGRAPHIC  DEVELOPMENT  BY  GAS  LIGHT. 
By  Wiwam  S.  Weakeey,  P.D. 
Before  entering  upon  the  practical  part  of  this  subject,  it  might 
be  well  to  first  consider  the  basis  upon  which  we  work  to  obtain 
certain  definite  results.  These  results  come  about  by  the  chemical 
action  of  light  rays  upon  the  photographic  dry  plate,  which  consists 
of  a  glass  plate  or  celluloid  (films)  coated  with  a  silver  bromide 
gelatin  emulsion. 
Upon  exposure  to  light  the  silver  bromide  particles  in  the  plate 
are  more  easily  converted  by  the  reducing  solution  (developer)  into 
metallic  silver  than  those  which  have  not  received  this  exposure. 
We  find  that  by  too  long  a  development,  or  by  using  too  strong  a 
developer  to  start  with,  the  unexposed  silver  bromide  is  also 
changed ;  for  this  reason  development  or  the  reduction  of  the  silver 
bromide  can  only  be  carried  on  to  a  certain  point. 
The  next  subject  to  be  considered  is  the  use  of  the  developer  or 
reducing  agent  which  brings  about  this  change.  These  agents  may 
be  divided  into  two  classes,  namely,  slow  and  rapid;  an  example  of 
the  former  class  we  find  in  hydrochinone,  and  of  the  latter  we  find 
in  pyrogallol.  In  using  a  rapid  developer  exposures  must  be  cor- 
respondingly correct,  for  if  they  are  not  the  reducing  solution  acts 
too  quickly  upon  the  unchanged  silver  bromide  and  hence  a  fog, 
or  as  expressed  by  Professor  Nipher,1  the  zero  point  is  ap- 
proached, if  not  already  reached.  With  a  developer  like  hydro- 
chinone in  its  normal  alkaline  combinations  we  have  a  typical  slow 
developer  whose  rapidity  is  materially  increased  by  replacing  the 
sodium  carbonate  by  potassium  or  sodium  hydrates.  This  de- 
veloper not  only  enables  one  by  its  delay  in  reducing  the  silver 
bromide  to  judge  an  over-exposure  and  remedy  it  by  potassium 
bromide,  but  also  assists  quite  materially  in  stopping  the  develop- 
ment at  the  proper  time,  thus  preserving  details. 
The  author  of  this  paper  had  his  attention  called  to  the  fact  that 
Prof.  Francis  E.  Nipher,  of  the  University  of  Washington,  was 
trying  to  turn  our  former  ideas  of  the  principles  of  photography 
upside  down,  and  at  the  suggestion  of  Professor  Kraemer  the  sub- 
stance of  Professor  Nipher's  paper  was  investigated  and  some 
1  "  Positive  Photography  with  Special  Reference  to  Eclipse  Work."  Pre- 
sented to  the  Academy  of  Science  of  St.  Iyouis,  October  15,  1900. 
