Am.  Jour.  Pharm. 
Feb..  1889. 
Photography. 
103 
It  will  appear  from  the  above  equations  that  the  chief  action  of  the 
developing  solution  is  to  rob  the  silver  salt  of  its  halogen,  and  thus  reduce 
it  to  the  metallic  state,  it  having  first  of  all  undergone  some  change  by  the 
action  of  the  light,  whereby  its  affinity  for  the  halogen  is  in  some  way  or 
other  impaired,  and  rendering  it  thus  more  easily  reduced.  This  reduc- 
tion must  take  place  in  a  very  gradual  manner  indeed,  or  the  result- 
ing negative  would  otherwise  be  very  harsh  and  utterly  devoid  of  that  beau- 
tiful gradation  and  softness  so  necessary  for  the  production  of  a  good  print ; 
hence  the  use  of  a  bromide  in  the  developer,  although  how  it  acts  as  a  re- 
strainer  is  not  exactly  known,  but  it  appears  in  some  way  or  other  to  assist 
the  silver  bromide  in  resisting  the  action  of  the  developer. 
Some  pyro  developer  having  been  made,  as  soon  as  night  sets  in  again 
light  the  ruby  lamp,  and  take  the  plate  on  which  the  three  exposures  were 
made  out  of  the  dark  slide.  On  looking  at  it  no  apparent  change  has  taken 
place;  it  appears  precisely  the  same  as  when  first  put  in ;  but  if  the  in- 
structions given  have  been  strictly  followed  a  faithful  representation  of  the 
subject  lies  latent  on  the  correctly- exposed  part,  and  the  next  step  is  to  make 
it  visible.  Placing  the  plate  in  a  small  dish  or  tray,  film  side  up,  pour  on 
the  developer  evenly,  gently  rocking  the  dish  to  and  fro  during  the  whole 
time.  Now,  supposing  two  seconds  to  have  been  the  correct  exposure,  then 
the  portion  of  the  plate  which  received  three  seconds  will  rapidly  flash  out, 
whilst  the  two  seconds  part  gradually  unfolds  itself,  first  the  high  lights,  as 
sky,  white  objects,  etc.,  and  then  the  detail  in  the  more  shaded  parts,  whereas 
on  the  portion  exposed  for  one  second  only  it  will  be  some  time  before  any 
change  at  all  is  perceptible,  then,  very  slowly  indeed,  the  high  lights  will 
become  visible,  but  there  will  be  no  detail  whatever. 
In  practice,  therefore,  as  soon  as  the  plate  has  been  flooded  with  the  de- 
veloper, it  is  a  very  easy  matter  to  tell  whether  the  exposure  has  been  cor- 
rect or  otherwise. 
As  soon  as  it  is  thought  that  all  the  detail  is  well  out  the  plate  must  be 
rinsed  in  a  little  clean  water,  and  put  into  the  fixing  bath,  which  is  another 
dish  containing  a  solution  of  sodium  thiosulphate. 
Immediately  the  creamy  appearance  (due  to  the  unaltered  silver  bromide) 
observed  at  the  back  of  the  plate  has  entirely  disappeared  this  operation  is 
complete,  and  it  only  remains  to  thoroughly  wash  the  plate  in  order  'to  re- 
move all  traces  of"  hypo,"  and  place  it  in  a  rack  to  dry.  The  result  is  a 
negative,  and  from  it  as  many  copies  as  desired  may  be  taken  by  one  of  the 
many  different  processes  of"  printing;"  for  instance,  it  can  be  done  either 
in  silver,  platinum,  uranium,  iron,  or  chromium  salts,  but  as  the  first-men- 
tioned is  the  oldest  and  most  popular  to-day,  I  shall  confine  my  remarks  to 
that. 
Ifc  Sod.  thiosulph, 
Aq.  ad  
Misce. 
3iv. 
■  Oj. 
{To  be  concluded.) 
