There  is  nothing  prohibitive  in  such  an  interval.  Indeed,  when  night  pho-
tographers often  give  an  hour  or  two  to  a  picture,  no  one  should  object  to  a  few
seconds,  or  even  minutes  for  pin  hole  work.  And  early  in  the  morning  in  the
winter,  if  working  in  shady  places  or  glades,  where  the  light  is  poor,  it  may  be
necessary  to  run  the  exposure  to  an  hour  or  more.  However,  for  such  exposures
the  best  advice  is  Punch's — Don't.  Wait  for  a  brighter  light  or  use  a  shorter
plate  distance.
Don't  forget  that  plate  distance  has  a  great  effect  upon  exposure.  Having
determined  the  proper  exposure  for  any  given  plate  distance,  don't  forget  that
the  formula  for  determining  exposure  for  other  plate  distances  is  the  same  as  for
a  lens — the  exposure  varies  as  the  square  of  the  distance.  If  you  need  twenty
seconds  at  eight  inches,  for  sixteen  inches  you  require  not  double,  but  four  times
that  exposure  with  the  same  pin  hole.  The  square  of  eight  is  sixty-four.  The
square  of  sixteen  is  two  hundred  and  fifty-six.  Two  hundred  and  fifty-six  is
four  times  sixty-four.  Therefore,  the  longer  plate  distance  requires  four  times
the  exposure  of  the  shorter.
