of an Arc of the Meridian. 391 
to the vertical microscope, is suspended a lamp on two arches. 
At the back of it is a hollow cylindrical recess, in which is 
placed a polished metallic segment of a sphere. This concave 
reflector is attached to the cylinder, by means which give it 
any position required ; so that the image of the burning wick, 
in the hollow of the lamp, may be thrown at pleasure on any 
spot above. 
From the end of the conical axis, on the same side with this 
lamp, projects a small brass arm, carrying at its extremity a 
speculum, whose surface is placed at 45 0 with the vertical, and 
directly opposite the open end of the sector's axis. When 
the image of the burning wick is thrown from the concave 
reflector on the flat one above, the light passes through the hollow 
axis, illuminating the mother-of-pearl, and is, at last, sent down 
the microscopic tube to the eye below. This contrivance, col- 
lectively taken, is unique, and is full as accurate in its operation 
as ingenious in itself. From its nature, granting perfection of 
work, there can be no parallax between the dot and the wire. The 
images of the illuminated circle and wire, (plumb-line,) are co- 
incident on the upper surface of the prismatic eye-glass, and 
transmitted so. In short, the whole has been so well managed, 
that the plumb-line can be made to bisect the dot or little circle, 
as accurately as the points on the divided limb of the sector. I 
consider this general description of this part of the instrument 
sufficient for the present ; the proper plate, and its appropriate 
explanation, will supply what yet remains to be said. 
The plumb-line is suspended above the upper part of the axis, 
from a point connected with the extremity of a bent lever, 
moveable round its fulcrum. The other end of the lever is acted 
on by a helical spring, which presses downwards, and causes 
' 3 E 2 
