COMMENTATIO ASTRONOMICA. 8J 



■ ■« II is impossible lo conceive a more simple process than thal of determining witli Ihe 

 -<( mural circle the difference of polar distance between these Stars. From Iheir proxi- 

 « mily in right ascension , the Operation is the same as tbat of measuring the angular dis- 

 '•« tance of two terreslrial objects, about 12° asunder, with a theodolite surrounded by 

 « sis microscopes : for the mural circle , in principle , cxactly resembles a verlical theo - 

 « dolite ; with this difference , tbat its microscopes , instead of being placed on a frame- 

 « work of brass, are securely fixed on a stone pier. Now I find thatthe angular distance 

 « thus measured in winter does not differ one-tenth of a second from the same angular 

 « distance measured in summer; and therefore, that the difference of parallax between 

 « the two Stars is absolutely a quantity loo small to be measured. In this investigation, 

 « it is to be considered that any constant error in the determination of the absoluie po- 

 « lar distances has nothing to do with *he question , it being the difference only of Ihose 

 « distances at opposite seasons that is required. To render all errors Ihroughout the 

 '« wbole course of Observation as constant as possible, the telescope remaiued fixed to the 

 « same part of the limb of the instrument , and the utmosl pains werej takeu to reciuce 

 « the:temperature in the Observatory to that of the ouler air." 

 fdem Pondius alio loco baec dixerat (*): 



- v a Lyrae and y Draconis have been observed logelher for five successive ycars. A- 

 « bove three hundred obscrvations of each star have been made in opposilion , and as 

 t< many in conjunclion , and I find the difference of parallax from the mean of all these 

 « observalions to be about 0,25", which quanlity by the Freach refraction would be re- 

 J( duced one-half, or lo on insensible quantity." 



Ad ea autem haec animadverlit Brinkleius ( f ) : 

 : « Now any person reading these passages mi^t understand , that Mr. Pond means , 

 « by the angular distance thus measured, the angular distance measured on the same 

 k day; not one slar observed on one day, and the other on another; and that in the 

 «■ latler passage he means the same by the words « observed togelher. "" 



- V Yet on examining the observations it will be found he cannot mean this, fov bis 

 « Table Xr. shows the contrary, the number of observations of each star being Ihere 

 « unequal; and it will also be found that the days in the five years (1812 to 1816 in- 

 « clusive) on which both slars were observed, only amounl to about 337, not recko- 

 ■« ning about 40 in 1812 and 1813, rejected or not used by Mr. Pond. Of these 337 

 « observations, 146 were in summer (in opposilion), and &7 i« winter (conjunclion) , 

 « and 100 in autumn ( in quadrature ). " 



« The» 



{ * ) Fhil. Trans. rSi?. P. I. p. ,€6. 

 <■ (t ) Phil, Trans. 1834. P. II, p, 473. 



H 



