550 
PROCESS  FOR  HELIOGRAPHIC  ENGRAVING. 
the  positive  pole,  you  eat  away  the  metal  at  these  points,  and  thus 
form  an  engraving,  so  that  you  can,  at  will,  according  to  the  bat- 
tery pole  which  you  use,  obtain  either  an  engraving  analogous  to 
an  ordinary  copper-plate,  wThich  can  be  printed  by  the  same  pro- 
cess, or  an  engraving  in  relief,  to  be  printed  like  a  wood-cut,  with 
printing  ink. 
When  it  is  desired  to  reproduce  an  ordinary  engraving,  clearly 
executed  upon  paper,  as  in  the  case  assumed  above,  the  photograph 
requires  no  peculiar  preparation  previous  to  its  transfer  to  the 
metal.  But  this  is  not  generally  the  object  of  photographic  en- 
graving ;  it  is  to  reproduce  natural  objects  without  any  intermedi- 
ate process.  When,  then,  it  is  required  to  reproduce,  for  instance, 
objects  of  natural  history,  landscapes,  or  monuments,  the  photo- 
graph used  must  be  got  in  a  way  somewhat  different  from  the  com- 
mon mode.  It  is  in  fact,  the  production  of  that  which  engravers 
call  the  grain,  that  is,  the  lights,  put  by  the  graver  in  the  shadows 
of  the  picture,  which  constitutes  the  essential  difficulty  in  engra- 
ving photographs.  The  photograph  has  nothing  of  the  kind,  for 
the  shadows  are  made  of  a  uniform  tint  ;  we  require,  therefore,  a 
peculiar  artifice  to  produce  this  grain,  which  does  not  exist  in  the 
photograph.  In  the  works  of  MM.  Rousseau,  Deveria,  and  Rif- 
faut,  they  are  produced  afterwards  on  the  plate  by  the  graver.  In 
the  new  process,  proposed  by  M.  Baldus,  this  grain,  so  necessary 
for  the  engraving,  is  formed  on  the  photograph  itself,  and  the  use 
of  the  graver  thus  rendered  superfluous.  This  grain  is  formed  on 
the  negative  photograph,  by  adding  to  the  sensitive  substances  a 
compound,  which,  crystallizing  in  the  paper,  forms  small  transpa- 
rent grains.  The  complete  publication  of  all  the  details  of  this 
new  process,  which  the  author  will  doubtless  make,  will  allow  the 
curious  chemical  effect  which  takes  place  under  these  circumstan- 
ces, to  be  understood. 
There  remains  but  a  single  word  to  be  added  :  the  proofs  on  pa- 
per obtained  by  these  new  plates  of  photographic  origin  are  so 
perfect,  that  the  great  problem  of  engraving  by  the  agency  of  light 
may  be  regarded  as  definitely  solved.  Not  only  this  new  process 
is  going  to  reduce  very  much  the  price  of  engravings,  but  there  is 
no  artist  or  amateur  of  photography,  who  may  not  soon  have  the 
pleasure  of  reproducing,  in  a  corner  of  his  room,  all  the  photo- 
graphs which  he  obtains. — Journal  of  the  Franklin  Institute  from 
Cosmos,  26th  May,  1854. 
