430 
Mr. Pond on the Delinations 
may be lost, for want of a corresponding one to compare with 
it. The mode which I propose to adopt to remedy these incon- 
veniences, will enable us to combine all the advantages of 
the two methods above described : it is extremely simple in 
its principle, and easy of execution, for it merely consists in 
uniting on the same plumb-line the two principles already 
explained. 
Two very fine holes should be made in the farther limb 
of the circle, and two lenses firmly fixed opposite to them, in 
the other, which should each form an optical image of its 
corresponding dot or hole, in the tube through which the 
plumb-line passes.* It will be best, if these dots are made 
exactly in a diameter, as they may then be used in two posi- 
tions. Beneath these should be formed the image of a lumi- 
nous point, according to Mr. Troughton’s present method, 
by an apparatus attached to the plumb-line tube ; when the 
two points on the circle move away, by the necessary opera- 
tion in observing, the lower point will remain stationary, and 
indicate any change of position in the whole instrument, if 
such should accidentally take place, and which by the other 
method alone would have passed unnoticed. 
The contrivance above described was executed for me at 
my request by Mr. Troughton, and is represented in the 
Plate ; but by some accident a part of the apparatus was 
* As these transparent dots are intended to be bisected by the plumb-line, they 
must be capable of the necessary adjustments, both for distinct vision, and for placing 
them in an exact diameter. 
It may be found more convenient in practice to arrange the whole apparatus in 
sliding tubes, but in whatever way the contrivance be executed, the points should 
ultimately be fixed as firmly as the divisions of the instrument. 
