^49 GKRA.R1)I REGNERI POCKEN S 



« pcd, tlial Ihe long and ledious exertions iliat have bccn iiscd in obtaining ihese re, 

 « sults, will not be found to have been enlircly without use." 



Prouti hoc loco suas maxtme tractavit observationes, ita mox , 14 Martii 1823, Pon- 

 •dianas diseussit (•) : 



« The opposite results deduced from tlie observations of Ihe Greenwich mural circfe 

 « and of the circle at ihe observatory of Trinily College, Dublin, relative to the pa- 

 »<^ rallax'cf cerlnin fixed stars , have induced me to examine more minutely the Green» 

 « wich observations. The apparent confirmalion of the observations made here, which 

 « I have rccenlly deduced from the determinalion of the solar nutation, seemed to ren- 

 .<i der the discordance of the instrumcnls still more remarkable. " 



« I h.ive been mach surprised to find tliat one of the simplcst methods of examining 

 -« the observations of llie mural circle appears to point out a parallax in c. Lyrae ncft 



V sensibly differinj; in quantity from that which I had deduced by the circle of the 

 « College Observatory. " 



« From the nature of the mural circle , the arch intercepted between the siafs ouglft 

 «.lobe found the same at all seasons of the year, provided the proper corrections be 

 « made for annual Variation , aberration , nutation and refraclion, and provided ihere 

 * be uo sensible parallax. If the arch be not found the same, some cause must exist , 

 t< eilher arising from the Instrument or some actual change in the relative position ' of 

 « the Stars. " 



« With a reference to the queslion of parallax, <x Lyrae has been chosen for exami'- 

 ti-nation, because it appears to have the greatesl parallax of any star near the zenill/. 

 « The pole-slar, when above the pole, from the great number of observations ihat are 

 (r to be found of it , from its having no sensible parallax aecording to my observations , 

 « and from some other circumstances , is an eligible star for comparison wilh x Lyrae'. 

 « üsing one star only as a point of comparison for other stars. feduces in some mea- 



V sure the tilüral circle to the simplicity of reversing instrumcnls, in which the zenilh 

 « point is determined by a plumb-line. The poIe-star bcsides is particularly pointed 

 t( out as' being Ihe ultimate object of reference for all stars in the use of the mural 

 v: circle. " 



« If the divisioKS of Ihe circle were perfectly exact , it would be immaterial whether 

 n< the position of the telescope remained the same at the opposite seasons in which the 



« ini 



(•) Memoirs of the Astronomical Socielj of london. Vol. I. P. II. iSaS. p Bsp. Tiluliis est; Th^ 

 results of computaüons relative to the parallax of a Lyrae from Ohservations made wilh the Green' 

 ■wich Mural Circle. By the B.ev. Dr. Brinkley, Andrews Professor of Asironomy in the Vniver- 

 •gily of Dublin , -F. B. iS. etci 



