AmMa"'iSiarm'}    Photographic  Development  by  Gas  Light.  235 
original  experiments  carried  out,  with  the  principles  therein  laid 
out  as  the  basis. 
We  have  in  the  sensitive  film,  three  stages  or  conditions,  namely  : 
the  negative,  zero,  and  positive  conditions. 
With  the  negative  stage  our  plate  is  exposed  the  normal  time, 
which  depends  upon  six  things. 
(1)  On  the  weather. 
(2)  On  the  brightness  of  object  to  be  taken. 
(3)  On  the  time  of  day  and  season. 
(4)  On  the  amount  of  light  transmitted  by  the  lens  used. 
(5)  On  the  size  of  aperture. 
(6)  On  the  sensitiveness  of  the  plate. 
These  six  conditions  with  the  dark  room  fix  the  basis  upon  which 
negative  photography  is  produced,  the  failure  to  take  any  one  of 
these  conditions  into  consideration  will  mean  failure  either  one 
way  or  the  other,  i.  e.,  undertimed  or  overtimed;  the  former  condi- 
tion meaning  a  thin  and  contrasting  negative,  the  other  a  dense  and 
non-contrasting  negative  or  fog;  this  fog,  when  perfect,  is  our  zero 
point,  or  where  the  negative  merges  into  the  positive  condition. 
Then  an  over-exposed  negative  may  be  an  under-exposed  positive, 
but  cannot  be  an  over-exposed  positive.  This  sufficiently  over- 
timed negative  or  positive  must  now  be  developed  in  the  light,  so 
as  to  carry  it  farther  and  farther  away  from  the  zero  condition  ; 
therefore,  the  nearer  the  zero  condition  is  approached,  the  stronger 
the  light  must  be  during  development,  so  as  to  carry  it  farther 
away  from  this  condition. 
The  application  of  positive  photography  is  obvious  when  we 
consider  the  liability  of  over-exposure,  especially  in  such  important 
work  as  eclipse  or  microscopic  photography  ;  think  for  a  minute  of 
the  occurrence  of  an  eclipse  which  perhaps  may  not  be  seen  again  for 
centuries,  and  the  application  of  this  new  process  will  be  apparent. 
Its  value  is  inestimable  when  we  consider  that  the  ordinary  nega- 
tive is  almost  invariably  over-exposed  for  fear  that  it  will  be  thin 
and  lacking  in  detail,  which  condition  in  a  negative  is  not  desired  ;  in 
fact  it  becomes  all  but  useless,  and  were  it  over-timed  and  devel- 
oped as  a  negative  the  mere  fact  that  potassium  bromide  would 
have  to  be  used  in  large  quantities  especially  in  greatly  overtimed 
plates  the  corresponding  result  would  be  lack  of  detail,  or  that  con- 
dition which  was  most  sought  for  is  destroyed  to  a  greater  or  less 
