PHILOSOPHICAL TRANSACTIONS. 
XIII. On the Results of Periodical Observations of the Positions and Distances of 
Nineteen of the Stars in Sir John Herschel’s Lists of Stars, favourably situated 
for the investigation of Parallax, contained in Part III. of the Philosophical 
Transactions for 1826, and Part I. 1827*. By Lord Wrottesley, F.R.S. Sfc. 
Received November 14, 1850, — Read January 16, 1851. 
In these communications. Sir John Herschel shows, that if the component members 
of a double star occupy a certain position with reference to the pole of the ecliptic, 
and one of them be supposed to be situated within a given distance from the earth, 
a change will be periodically produced in the angle of position, and in the distance of 
the two stars forming the double star, consequent on the motion of the earth in her 
orbit ; that the maximum of the change in the angle of position will occur at two 
periods of the year distant from each other about six months ; and he gives formulae 
by which the epochs at which that maximum occurs may be computed, and by which 
the amount of parallax due to a given change in the angle of position may be also 
found for each star ; also lists of stars favourably situated for the investigation of that 
element, with the times of the year at which they should be observed, and the amount 
of parallax, which an observed change of 30' in the angle of position would indicate 
in the case of each star. 
On the erection of my observatory in the autumn of 1842, comprising among its 
instruments an equatoreal of very considerable power, I determined to employ it in 
the observations of the double stars contained in these lists, with the view of ascer- 
taining whether they exhibited such decided differences in their positions, when ob- 
served at the proper periods, as to give good grounds of hope that some definite 
conclusion might be arrived at, as to the existence of a parallax in the objects ob- 
served, capable of being measured, or at the least, confidently announced as subsist- 
ing in fact. 
* I have to acknowledge my obligations to Mr. Main of the Royal Observatory, Greenwich, for many valuable 
suggestions during the preparation of this communication for the press. 
2 X 
MDCCCLI. 
