46 
Photography. 
Am.  Jour.  Pharm. 
Jan.,  1889. 
graphy  as  the  period  that  marks  its  birth,  and  when  Daguerre,  a  poor  French 
painter,  first  exhibited  to  the  gaze  of  an  astonished  world  the  great  discov- 
ery which  now  bears  his  name,  the  daguerreotype,  its  appearance  being 
warmly  welcomed  by  the  vast  majority  who  saw  at  last  a  long  felt  want  sup- 
plied, the  very  small  minority  consisting  of  those  who  gained  a  pittance  by 
the  art  of  miniature  portrait  painting,  and  to  whom  it  was  painfully  appa- 
rent that  their  occupation  had  fled.  The  French  government  then  granted 
the  father  of  photography  and  his  clever  partner,  Isidore  Niepee,  a  pension, 
chiefly  on  the  ground  that  "  The  invention  did  not  admit  of  being  secured 
by  patent,  since,  as  soon  as  published,  all  might  avail  themselves  of  its  ad- 
vantages ;  they  therefore  preferred  to  enjoy  the  glory  of  endowing  the  world 
of  science  and  art  with  one  of  the  most  surprising  discoveries  that  honor  their 
native  land." 
In  England,  Wedgwood,  bearing  in  mind  the  experience  of  Scheele  with 
silver  chloride,  and  assisted  by  Sir  Humphry  Davy,  had  been  busily  em- 
ployed in  conducting  experiments  at  the  Royal  Institution  with  this  sub- 
stance ;  but,  although  he  had  actually  succeeded  in  obtaining  images,  no 
method  could,  by  any  possible  means,  be  discovered  of  "  fixing  "  or  making 
them  permanent,  so  that,  when  exposed  to  the  light,  his  pictures  gradually 
faded  from  view. 
Now,  there  is  here  real  reason  for  regret,  because  a  year  or  two  later,  be- 
fore the  discovery  of  the  daguerreotype  had  taken  place,  Sir  John  Herschel 
first  prepared  and  demonstrated  the  properties  of  hyposulphite,  or  more 
properly  speaking,  thiosulphate  of  sodium,  particularly  calling  attention  to 
the  remarkable  solubility  of  silver  chloride  in  its  solution.  Here,  then,  was 
the  very  thing  for  which  Davy  had  longed  in  vain,  and  it  is  now  generally 
supposed  that  Herschel  was  ignorant  of  the  results  obtained  and  published 
a  few  years  previously  by  Wedgwood  and  his  illustrious  assistant,  Davy.  An- 
nother  circumstance,  perhaps  still  more  remarkable,  is,  that  even  Daguerre, 
when  he  made  his  great  discovery  known,  was  not  in  possession  of  any  per- 
fect method  of  fixing  his  pictures,  and  it  thus  remained  for  Herschel  to  once 
more  call  attention  to  his  discovery  as  the  only  means  by  which  photographs 
on  silver  salts  could  be  permanently  "fixed." 
Herschel,  also,  first  suggested  the  use  of  glass  plates,  and  his  cyanotypeis, 
even  now,  largely  used  by  draughtsmen  and  others  for  taking  copies  of 
drawings,  tracings,  etc.  In  this  process  one  side  of  a  sheet  of  white  paper 
is  brushed  over  (in  the  dark  or  some  non-actinic  medium),  with  a  solution 
made  by  dissolving  1  ounce  each  of  potassium  ferricyanide  and  ferric  ammo- 
nium citrate  in  8  ounces  of  water.  When  dry  it  is  ready  for  use,  and  may 
then  be  exposed  to  a  good  light,  beneath  the  subject  to  be  copied,  and  after- 
wards developed  "  by  simply  washing  in  plain  water.  During  the  process 
of  printing  the  ferric  salt  is  reduced  to  ferrous  by  the  action  of  the  light  (in 
the  presence  of  organic  matter),  and  this  at  once  combines  with  the  ferri- 
cyanide present,  to  form  ferrous  ferricyanide  or  Turn  bull's  blue,  which,  be- 
ing insoluble,  is  not  affected  by  the  subsequent  washing  in  water,  whilst  the 
unchanged  ferric  salt  is  easily  washed  out,  and  thus  an  exact  facsimile  of 
the  subject,  be  it  negative,  drawing  or  tracing,  remains. 
