the Pendulum vibrating Seconds in the Latitude of London. 179 
and adjoining was a piece of black paper to prevent the reflec- 
tion of light on the knife edge from the surrounding objects. 
The knife edge now appeared through the microscope, as a well 
defined dark object on a white ground. 
Marks were made on the paper close to the knife edges at 
equal distances on each side of the bar. These were intended 
to indicate those parts of the knife edges equally distant from 
the middle, from which the measurements were to be taken. 
The knife edges being adjusted parallel to each other, in the 
manner before described, the microscopes were brought succes- 
sively over such marks on the paper, as were at the same distance 
from the bar; and the mean of each pair of observations being re- 
ferred to the scale, gave a distance of the knife edges free from 
any error which would be occasioned by a want of parallelism. 
The knife edges bisecting the cross threads of the micro- 
scopes, pieces of black paper were slid beneath them, when they 
appeared to start forwards towards each other, the images con- 
tinuing perfectly sharp and well defined. 
~ The distance between the knife edges appeared to be now con- 
siderably less than before; and it remained to determine tue 
difference, in order to apply its half, as a correction to the di- 
stance 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 cn a black 
ground. ‘The difference of such readings will obviously give 
double the correction required. The results are contained in 
the following table. 
| } 
IDivisions of{ Divisions ii 
ithe Microm.|the Microm. 
|the ground |the ground 
\being white.|being black. 
| 
Difference. 
39-0 | 44-0 | 12-00 | 
19:5 | 300 | 10:50 
17-5 | 280 | 10:50 
5 
16: 27-7, |. 11-20 
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°7 165 | 10-80 
5-0 165 | 11:50 
i ~~ Mean} 11.03 
From 
ee er 
