COMMENTATIO ASTRONOMICA. 543 



« equally satisfactory; for amoiig Ihe Observation of this star we may find occasional 

 « discordaiices that admit of being interpreted in favour of parallax. And althou^h I 

 « have been incliiird myself to atlribiite these irregularities lo olber causes , yet Iheir 

 « existence made it desirable to inslitiite new esperiments. The mclhod with a fixed 

 K telescope, -which I had contrived for » Cygni , could not bere , I found , be applied 

 « snccessfully : there being no star of nearly the same' al itiide but opposile , in right 

 • ascension sufficienlly bright tö be observed Ihroughout the year , a circnmstance quite 

 « essential to that mode of Observation. I have employed therefore the mural circle lo 

 « investigate; Ist, the difierence of parallax belween y Draconis and «Lyrae: 2dJy, 

 « the absolute parallax of the latter s!ar: the Dublin observations indicatiiig, it inay 

 « be remembered , that the parallax of 7 Draconis is insensible, but that of a Lyrae a 

 « very perceplible quantily. The processes employed in these Iwo investigalions beiag 

 « very diffcrenf, I shall consider each of them separaleiy." 



1.) Quod ad differenliam parallaxeos Draconis </ et Lyrae a, altinet: 

 <i . . . I find that the angular distance in winter does not difFer one-tenth of a se- 

 K cond from the same angular distance measui-ed in summer; and therefore , that the 

 « difference of parallax between the two slars is absolutely a quantity too small lo be 



V measured. In this investigalion , it is lo be considered that any conslant error in Ihe 

 « delermination of the absolute polar distances has nothing lo do with the question, it 

 « being the difference only of those distances at opposile seasons, Ihal is required. 



« To Tender all errors Ihroughout the whole course of Observation as conslant as pos» 

 «r sible, the telescope remained fixed to Ihe same pari of the hmb of the instrument, and 

 « tlie Ulmost pains were taken to reduce the temperature in the observatory to that of 

 « the ouler air ; the difference Ihroughout the year not exceeding one degree. " . . . . 



Enumerat merita circuli Grenovicensis et circuli Dublinensis: cautionesque memorat 

 delicatissimas , quas ad observaliones suas adhibuerit. 



Caeterum autem « the preceding observations only indicate that y Draconis and 



V a Lyrae have the same parallax , or that their difference of parallax is zero ; but ihey 

 « have no tendency to show what is the actual magnilude of Ihe parallax that the two 

 « Stars have in common. If indeed we admit it lo be proved , by the observations of 

 « Bradley, and Ihe more recent ones of Dr. Brinkley, that the parallax of y Draconis 

 « is insensible, we may then infer from the observed difference what is the parallax of 

 « the olber star. But the melhod of investigation that we are now about to consider , 

 « does not depend on such an admission. " 



2. ) Quod ad absolulam altinet Lyrae a parallaxin : 

 , • Having successfully adopted Ihe meihod of observing by reflection, I was desirous 

 c of employing it in a series of observations upon « Lyrae , with a view to dclermino thi» 



H h 2 « qiies- 



