the trigonometrical Operation * 43 
triangles were obferved, and thereby it was eafily feen in the 
courfe of the operation, whether the inflrument had fuffered 
any change to render a re-adjuftment neceffary. 
Art. X. Lanterns for the Illumination of the IV ires. 
The axis of the tranfit telefcope rs*hollow, and in the middle 
there is placed, at an angle of 45 0 with the axis of vifion, a per- 
forated elliptical illuminator for throwing light on the wires in 
night obfervations. The light is communicated from a fmall 
lantern attached to the horizontal bar at its jundlion with the 
brace, direflly oppofite to the end of the axis, which has a bit of 
thin glafs placed before it to prevent duft from entering. There 
is another fuch lantern for the lower telefcope, not however 
reprefented in the plate. As the light given by thefe lan- 
terns was found to be rather too weak, efpecially that for the 
upper telefcope, therefore it was cuftomary in pradlice to .illu- 
minate the wires, by holding up frontwife one of thofe feen in 
the fedlion in Plate IV. againft the end of the axis of the upper 
telefcope, when directed to the pole ftar. The fame method was 
ufed by prefenting it obliquely to the object-glafs of the lower 
telefcope, when it became neceffary to examine whether the 
interfe£lion of the wires continued without fenfible variation 
on a reverberatory lamp, commonly placed twelve or fifteen 
miles off, and fometimes even at the great diflance of twenty 
or twenty- four miles. 
Art. XI. Lanterns for throwing light on the Divifions of the 
infrument . 
Befides the two fmall lanterns for illuminating the wires of 
the telefcopes in night obfervations, two larger ones may be feen^ 
as already mentioned, {landing on the mahogany cover in the 
fe ci ion 
