of an Arc of the Meridian. 399 
or small circle e, described on the thin slice of mother-of- 
pearl, shown in the section of the axis at e. 
A lamp is attached, or rather rests, on circular supports an- 
nexed to the side of the interior frame, and may be seen at XZ. 
At the back of the lamp, placed in a recess, is a concave reflector 
at Z ; and, in the front of it, a tube running out to X, having a 
double convex glass at P, for throwing the light on L, which 
first passes through a double concave glass in the side of the 
telescope, and then, from the reflector L, is thrown down on the 
wires near K. The concave speculum Z, has two adjustments for 
converging the reflected light on the little elliptical speculum h. 
This last-mentioned speculum throws off the said light at h, 
which passes into the axis at G, illuminating the mother-of-pearl 
at e, and, finally, is transmitted out of the axis at p . 
klmn, is a section of the long microscope, for conveying the 
image of the dot and wire, sent out of the axis at p, to the eye at 
k. This microscope is firmly attached to the side of the frame, 
by brass cylinders, kk, //, mm, n, and has one plano-convex 
glass at q, a prismatic eye-glass at u, and a metallic reflector at 
the top, 0. At the upper end of this long microscope, and 
directly behind the speculum, is a screw, by which the reflecting 
metal is brought into one of its requisite positions. The other 
adjustment of that metal is performed by two screws, which 
apply to the sides, and give it lateral motion. The plano-convex 
glass at q, is rivetted into the head of a long tube uq , which slides 
up the microscope. The upper part of the microscope at 0 , is 
placed exactly opposite the end of the axis, in a very firm way. 
The rod for giving motion to the plumb-line, is vwx; v 
being the top of it, w the place of the universal joint, which 
separates the two parts of the rod, and x the bottom of the rod 
3 F 2 
