550     PROCESS  OF  TAKING  PHOTOGRAPHIC  PICTURES  ON  GLASS. 
face,  upon  which  it  forms  a  colorless  transparent  film  ;  the  excess 
of  collodion  is  then  allowed  to  run  off,  and  the  glass,  being  still 
held  horizontally,  is  inclined  to  one  side  and  the  other,  until  the 
collodion  becomes  partially  thickened  or  set.  When  this  has  ta- 
ken place,  and  before  it  is  dry,  it  is  rinsed  in  a  solution  of  crys 
tallized  nitrate  of  silver,  of  a  strength  of  forty  grains  to  tho  ounce 
of  water ;  the  film  is  thus  impregnated  with  iodide  of  silver,  and 
after  remaining  in  this  bath  a  sufficient  length  of  time  for  the 
ether  to  escape  from  the  collodion,  the  plate  is  ready  to  be  placed 
in  the  camera.  After  being  exposed  a  sufficient  length  of  time 
in  the  camera,  it  is  taken  to  a  dark  room,  where  the  latent  pic- 
ture is  developed,  by  the  application  of  a  solution  of  protosulphite 
of  iron,  acetic  acid,  and  nitric  acid,  in  about  the  following  pro- 
portions : — One  quart  of  soft  water,  one  ounce  protosulphite  of 
iron,  thirty-two  drachms  No.  8  acetic  acid,  one  drachm  nitric 
acid.  These  exact  proportions  are  not  rigid,  but  I  have  found 
them  to  be  sufficient  for  the  purpose  of  developing  the  picture. 
After  this  is  accomplished  it  is  washed  in  clean  soft  wTater,  and 
then  the  remaining  iodide  of  silver  is  dissolved  from  the  collodion 
by  a  solution  of  hyposulphite  of  soda,  after  which  the  picture  is 
entirely  cleansed  of  the  hyposulphite  solution  by  washing  as  be- 
fore, in  soft  water.  The  picture  is  then  dried,  either  in  the  open 
air  or  by  the  aid  of  a  gentle  heat,  and  the  process  is  completed. 
To  permanently  improve  the  beauty  of  the  pictures,  and  to 
deprive  them  of  a  bluish,  hazy,  indistinct  look,  is  the  object  of 
my  third  improvement  t  which  consists  in  the  application  of  a 
coating  of  balsam  of  fir  to  the  surface  of  the  glass  upon  which  the 
picture  is  made,  the  balsam  being  confined  to  the  picture  plate  by  a 
secondary  plate  of  glass,  which  is  applied  to  the  picture  plate  in 
a  manner  which  will  now  be  described,  and  which  hermetically 
seals  up  the  picture  and  protects  it  from  every  and  any  injury 
not  sufficient  to  fracture  the  glasses  themselves.  This  part  of 
the  process  will  now  be  described. 
A  second  plate  of  glass  is  prepared  of  the  same  size  as  that 
which  carries  the  picture,  and  is  thoroughly  cleansed  ;  the  pic- 
ture plate  is  then  held  horizontally,  the  picture  side  uppermost. 
The  balsam  is  then  applied  in  a  line  along  one  edge  of  the  glass, 
and  one  edge  of  the  secondary  plate  is  then  applied  to  the  edge 
of  the  first,  which  contains  the  balsam.    The  two  plates  are  then 
