length of the pendulum vibrating seconds. 59 
scopes, pieces of black paper were slid beneath them, when 
they appeared to start forwards towards each other, the 
images continuing perfectly sharp and well defined. 
The distance between the knife edges appeared to be now 
considerably less than before ; and it remained to determine 
the difference, in order to apply its half, as a correction to the 
distance first obtained. 
For this purpose, the reading of the micrometer was taken 
when the knife edges were viewed as dark objects on a white 
ground, and also when they were seen as light objects on a 
black ground. The difference of such readings will obviously 
give double the correction required. The results are contained 
in the following table. 
Divisions of 
the microm. 
the ground 
being white. 
Divisions of 
the microm. 
the ground 
being black. 
Difference. 
32,0 
44 ,° 
12,00 
1 9,5 
30,0 
10,50 
' 7,5 
28,0 
10,50 
2 7>7 
1 1,20 
12,5 
25,0 
12,50 
12,5 
22,0 
9,50 
12,0 
23,0 
11,00 
10,0 
21,0 
11,00 
9,7 
18,0 
8,30 
5,5 
19, 0 
13,50 
5,7 
16,5 
10,80 
5,o 
1 ^,5 
11,50 
Mean 
11,03 
From the above table it seems that 5,51 divisions (or 
,000236 of an inch) are to be subtracted from the distance 
