38 M. O. A. Rosenberger on the Elements 



of Lord Macclesfield, at which Bradley often assisted; but he 

 could not say whether the second observation on May 25 was 

 made by Bradley, or not." 



The star, with which the comet was compared from May 

 16th, is in the Fundumenta. I have corrected the places of 

 the comet, which depend on it, so that they may agree with 

 the Fundaynenta, the apparent place given to it above being 

 considered as the foundation of Bradley's reduction. 



5. Hell communicates both his original observations and 

 their reduction in his Ep/iemeris for 1760, p. 6. The reduc- 

 tion is often distorted by all kinds of errors of calculation, 

 which I have rectified. He observed alternately with two differ- 

 ent screw-micrometers and a reticule of forty-five degrees. 

 One of the wires was always placed parallel with the motion of 

 the comet or of the compared star, as soon as one or the other 

 entered the field of view. Of his observations he communi- 

 cates only those which were most correct, but he himself values 

 their accuracy only at from one to two minutes of space. 1 have 

 only communicated and taken into account the mean of his 

 observations for every day. There is only one observation 

 given for the 28th of May, which he considers very uncertain, 

 and almost half guess. 



6. I have treated the last class of observations mentioned 

 together, as if they belonged to one observer. The observa- 

 tion of Darquier is found in his Observations, i. p. 28. La- 

 caille gives a lonjj series of observations in the Mem. de 

 VAcad. An 1760, p. 58, but only the first three are given in 

 detail. Those of the Leyden astronomer Lulofs are also found 

 there, p. 440, with a reference to the fifth volume of the Me- 

 moirs of the Academy of Haarlem. But this treatise, of which 

 Prof. Harding had the goodness to furnish me with a written 

 copy, contains no more, essentially, than the extract in the Pai'is 

 Memoirs, for which reason I was obliged to exclude some 

 suspicious micrometer observations. Lastly, the observations 

 of Le Seur and Jacquier at Rome are given by Lalande in his 

 Histoire de V Acad, An 1759, p. 153, as very valuable. 



We may assume, even where it is not expressly stated, 

 though it is done more or less distinctly by most observers, 

 that one wire of the micrometer was placed parallel to the ap- 

 parent motion of the fixed star. The influence of refraction 

 has, therefore, been every where calculated according to the 

 formula (3) of the Astron. NacJir. vol. iii. p. 386. It is however 

 for the most part quite beneath consideration. 



In order to employ Encke's method with facility {Zeitschrift 

 fur Astron. Band vi, p. 151), in the formation of the final 

 equations furnished by the method of least squares, the vari- 

 ations 



