36 



ni, by 47 observations = 2'', i.* The results as to other 

 stars I shall reserve till my observations have been more 

 numerous. 



I shall endeavour to make such remarks respecting these 

 results, that the Academy may form a proper judgment how 

 far they serve to- establish the important conclusion, which 

 I conceive may be derived from them. 



For this purpose it is necessary to give some account of 

 -the instrument by which the observations have been made. 



The superiority of circles over quadrants, and the advan- 

 tages derived from the micrometer microscope, are so well 

 known, as to make it quite unnecessary, that they should be 

 -Stated here. 



About the year 1788, soon after the late celebrated artist 

 Mr. Ramsden had strenuously recommended those improve- 

 ments, which his mechanical skill had rendered so very prac- 

 ticable, the Provost and Senior Fellows of Trinity College, 

 Dublin, by the advice of my predecessor, Dr. Usher, di- 



* Perhaps it may be objected to me that what I call parallax, should be called double 

 parallax, since properly the parallax is the diflference between the observed and mean 

 place. In this way the 



parallax of a, Lyrx = 1",0 

 of a, Aquilae = 2,7 

 of Arcturus = 1,1 

 of a Cygni = 1,0 



But I believe that, generally, when the parallax of a fixed star is spoken of, the 

 greatest change of place is intended, which is equal to the angle subtended by the dia- 

 meter of the earth's orbit at a fixed star. 



