of the Principal Fixed Stars. 127 



the observers. If the subject be rightly considered, I doubt 

 not It will plainly appear that these instruments are not, in the 

 smallest degree, answerable for the discordances. The splen- 

 did talents of Messrs. Ramsden, Troughton, and Reichenbach 

 to whom we owe the construction of these instruments, have 

 been most strikingly exhibited therem. It is from another 

 source that the disagreement of the respective results is to be 

 sought for. 



I think it will be found that errors in the quantity of the 

 constaiit of refraction have occasioned the whole difficulty. I 

 shall endeavour to show, that an error of about half-a-second 

 in this quantity, as used by the different observers, will produce 

 very nearly the whole effect. Also that, apparently, the at- 

 tempt to ascertam the constant of refraction to this degree of 

 precision, is one of the most difficult things that have yet been 

 auned at m astronomy. It appears to me that no observer, 

 however excellent his instrument, and great his care and skill, 

 can he certain that he has arrived at this degree of exactness. 

 It might be inferred from this, that I suppose the errors of 

 the results may be equally divided between the Koniasberff 

 catalogue, and those of Greenwich and Dublin. It may not 

 be very unsafe to rest on this conclusion. But, fi-om a circum- 

 stence arising out of a recent comparison of the catalogues 

 of Mr. Pond and Dr. Brinkley, a degree of probability is ob- 

 tained, that their catalogues are the most exact. 



For the purjDose of makmg a comparison with M. Bessel's 

 catalogue, I shall use the recent determinations by Dr. Brink- 

 ley ot North Polar distances, which were published in the 

 Joiirnal of Science in September last, and refer to the last 

 number of the Philosophical Magazine for M. Bessel's de- 

 dmations. I might equally have used Mr. Pond's North Polar 

 distances; but, as the mean difference of Dr. Brhikley's North 

 Polar distances of 1813 reduced to 1820, and of those in the 

 Journal of Science, is precisely = 0",0, I have taken the latter. 



M. Bessel appears to have examined, with great care and 

 industry, the divisions of his circle; and there can be no rea- 

 son tor not acquiescing in his opinion of the excellence of his 

 msrument in this respect. Unfortunately, M. Bessel has not 

 added tlie decimations of stars between his zenith and the Pole • 

 with the assistance of these, we might have made an interest- 

 ing comparison of corresponding large arches in each circle. 

 A comj^arison of this kind has been recently made, as to the 

 circles of Dublin and Greenwich; and the result has been 

 such, as to leave no manner of doubt that the divisions of these 

 instruments agree together m a very exact manner; and ap- 

 pears 



