364 Mr. Nixon on the Verification of Captain Lloyd's 



latory theory of light, time alone can determine, and some 

 relation must be established between the compressing force 

 and the breadth and forms of the rings before we should be 

 warranted in deducing any conclusions. 



I am, Gentlemen, yours, &c. 

 Liverpool, Sept. 12, 1835. A. R. 



XLIV. On the Verification of Captain Lloyd's Levelling In- 

 strument by the Greenwich Mural Circle. By John Nixon, 

 Esq* 

 T N order to prove that the gun-metal collars of the superb 

 T level, by Cary, employed by Captain Lloyd in levelling 

 from London to Sheernessf, had been constructed of equal 

 diameter, the instrument was set up in the north window of 

 the Observatory at Greenwich, at a distance of about eight 

 feet from the mural circle J. The cross wires of the (thirty- 

 inch) telescope of the level being adjusted to the sidereal focus, 

 by observing when those of the great telescope of the circle, 

 as seen against a disc of white paper placed about an inch 

 from its eye end, appeared perfectly distinct, the horizontal 

 wire of the level was adjusted for collimation, and the great 

 telescope fixed at the (mean) horizontal point of the circle, 

 (which point bisects the arc passed over by the telescope in 

 observing the direct and reflected altitude of a star). On 

 looking through the telescope of the level, its horizontal wire 

 was found to conceal or cover that of the circle so completely 

 that the wires could not be seen separate without a slight ele- 

 vation of the telescope of the level. In confirmation of the 

 correctness of the observation, the great telescope, previously 

 inclined, was moved until its horizontal wire coincided pre- 

 cisely with that of the undisturbed level. On reading off 

 the microscopes of the circle, the direction of its telescope 

 proved to be horizontal within a small fraction of a second. 



The success of the method must evidently be in proportion 

 to the degree of fineness of the thicker of the two horizontal 

 wires made use of; because, as both may be considered 

 opake, it would be impossible to distinguish with what part 

 of the diameter of the thicker wire the finer one coincided, 

 or, indeed, whether they were merely sufficiently approxi- 

 mated not to appear separate. The wires of the Greenwich 



* Communicated by the Author. 



f See the Philosophical Transactions for 1831. [or Phil. Mag. and 

 Annals, N.S., vol. ix. p. 368.— Edit.] 



% H the measurement from the object-glasses or from the centres of 

 the two instruments ? 



