750 
Dr.  E.  S.  Johonnott  on  the 
Table  II. 
A. 
B. 
Limiting  Thickness  of  First 
Black  Film. 
Mean  Thickness  of  the  Second 
Black  Film. 
11-05,  1220,  12-20, 
13-15,  11-10,  12-30.. 
11-75,  1260,  13-30. 
6-03,  6-24,    *9-25,  5'52,  6-17, 
5-05,  6-28,    *7-30,    *9-50, 
5-40,  6-40,    *8-0. 
Mean    1218. 
Mean  6'76. 
It  would  be  difficult,  in  section  B,  to  distinguish  between 
the  values  corresponding  to  the  unstable  condition  and  the 
condition  of  limiting  thickness.  The  larger  values,  marked 
with  the  asterisk,  were,  however,  in  all  cases  followed  by 
smaller  values  included  in  the  table.  If  we  take  the  mean 
of  the  values,  not  including  these  larger  values,  to  determine 
the  limiting  thickness  of  the  second  black,  we  obtain  5*88  /jl/j*. 
This  would  indicate  that  the  limiting  thickness  of  the  second 
black  is,  approximately,  half  that  of  the  first.  No  very  great 
thickness  was  measured  in  the  first  black,  since  it  was  impossible 
to  count  the  coloured  fringes  for  such  large  deflexions. 
Variable  Atmospheric  Pressure. — The  following  readings 
were  obtained  when  changing  the  pressure  on  the  atmosphere 
surrounding  the  films. 
Table  III. 
Time. 
Fringes 
deflect. 
JNo. 
films. 
Mean 
thick. 
Temp. 
°  O. 
Pressure 
change. 
Remarks. 
A.M. 
fXfl 
cms.  Hg 
9.08 
films 
formed. 
17-8 
9.32 
30 
101 
26-8 
20-0 
none. 
All  first  black. 
9.50 
23 
100 
20-3 
20-0 
10.20 
2-0 
98 
18-0 
21-0 
-4-6 
tt           it 
3-4 
98 
306 
+9-2 
All  second  black. 
10.23 
•9 
98 
8-1 
21-6 
5>                             J> 
System 
allowed 
to  com 
e  to  no 
rmal  pres 
sure. 
10.40 
2-0 
97 
18-2 
none. 
-6-4 
All  first  black. 
4-2 
96 
38-5 
+12-8 
j>           >» 
10.44 
•9 
95 
8-4 
21-2 
All  second  black. 
