748  Dr.  E.  S.  Johonnott  on  the 
optical  paths  should  lie  alike  in  the  atmosphere  of  the  films. 
The  long  brass  box  in  which  the  films  were  formed  is  shown 
at  B.  The  films  were  221  in  number  and  formed  on  frames 
of  glass-fibre  as  in  the   former  work.     The  box  was  sealed, 
Fig.  2. 
ill 
and  connected  to  a  manometer  and  suitable  apparatus  for 
varying  the  pressure.  The  fringes  were  viewed  with  a  tele- 
scope. All  other  details  were  much  the  same  as  in  the  earlier 
work,  and  may  be  found  described  therein. 
At  the  first  trial  it  was  apparent  that  the  reading  of  the 
number  of  fringes  displacement  could  not  be  made  as  promptly 
as  the  thickness  could  be  altered  by  varying  the  pressure. 
This  was  due  to  the  distortion  produced  in  the  fringes  by 
quickly  changing  the  pressure.  It  was  very  difficult  to 
obtain  trustworthy  data  when  working  with  rapid  changes  in 
the  pressure,  and,  on  the  whole,  this  part  of  the  work  was 
somewhat  disappointing.  Nevertheless,  with  less  rapid  alter- 
ations in  the  pressure,  it  was  possible  to  pass  the  black  film 
through  all  its  phases  with  little  if  any  distortion  in  the  fringes. 
A  large  portion  of  the  time  was  spent  in  working  with  no 
variation  in  the  pressure.  The  method  usually  employed 
was  to  form  the  films  in  the  morning  and  take  readings  of 
the  thickness  throughout  the  day,  with  a  more  or  less  rapidly 
rising  temperature.  The  box  was  surrounded  w-ith  a  large 
water-jacket  to  maintain  a  uniform  temperature.  It  was 
found  that  this  outer  box  filled  with  air  served  the  purpose 
quite  as  well. 
The  following  table  wras  taken  with  no  variation  in  pressure. 
The  box  containing  the  films  was  firmly  sealed.  The  tem- 
perature recorded  was  the  mean  of  the  readings  of  two 
thermometers,  placed  within  the  box,  B. 
