[ '5 ] 



rcprefented Fig. III. In this, the centre of motion of the arm 

 which carries the counterpoife and lantern does not lie in the 

 axis of the inftrument, but at fome diftance above it, by which 

 means the axis is difengaged from furrounding wood-work, which 

 is an obflrudiion to the facility of reverfing the inftrument for 

 adjufting or verifying the line of collimation ; and that the light 

 in the lantern may move in an arc equal and parallel to that 

 defcribed by the elliptic illuminator, the light muft be as far 

 diftant from the point of fufpenfion of the lantern, as in the 

 centre of motion of the arm above the axis ; in this method, 

 therefore, a lamp or a candle with a fpring focket muft be 

 ufed. 



In Fig. I. and III. the two fpecies of elliptic illuminators, ufu- 

 ally applied, are reprefented ; that in Fig. III. is an elliptic ring 

 of pafteboard or other proper fubftance made white ; the left'er 

 axis of the ellipfc is fomewhat lefs than the diameter of the ob- 

 jedt glafs , — the other fliown in Fig. I. is a fmall cylinder of ivory 

 placed oppofite the centre of the objed glafs : In each the fur- 

 face is fet in an angle of forty-five degrees to the axis of the 

 telefcope. Dodor Mafkelyne, whom I believe the inventor of 

 the folid illuminator, has improved it lately, by fubftituting for 

 the ivory furface a fmall refleding mirror, by which he obtains a 

 fufficient quantity of light for the wires, without fo much lofs of 

 the central rays. 



In our tranfit inftrument the entire light of the objcd glafs is 

 preferved by a moft fimple contrivance : — That pivot of the axis 

 which refts upon the plate that regulates the motion in azimuth, 



is 



