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Observations of the apparent Distances and Positions of 380 Double 

 and Triple Stars, made in the Years 1821, 1822, and 1823, and 

 compared with those of other Astronomers ; together with an Ac- 

 count of such Changes as appear to have taken place in them since 

 their first Discovery. Also a Description of a Five-feet Equatorial 

 Instrument employed in the Observations. By John Frederick Wil- 

 liam Herschel, Esq. F.R.S. and James South, Esq. F.R.S. Read 

 January 15, 1824. [Phil. Trans. 1824,^. 1.] 



The determination of the apparent distances and positions of such 

 double stars as could be measured with micrometrical instruments 

 and high magnifying powers, was suggested by Sir William Herschel 

 more than forty years ago, and in his hands it led to a new depart- 

 ment of physical astronomy, by the discovery of sidereal phenomena 

 referrible to the agency of attractive forces ; but the determination of 

 the existence of annual parallax, the immediate object for which the 

 inquiry was instituted, was soon lost sight of in the more extensive 

 views of the construction of the universe, which gradually unfolded 

 themselves. Nor has the investigation been resumed, although from 

 the precision with which such observations can be made, it seems, in 

 the opinion of the authors of this paper, likely to be the mode by 

 which the existence or non-existence of sensible parallax will ulti- 

 mately be determined. 



The results of Sir William Herschel's observations, from 1779 to 

 1784, were published in the Philosophical Transactions from 1782 to 

 1 785 ; and a re- examination, after a lapse of twenty years, was under- 

 taken by him in 1801, -2, -3, and -4; and in the Transactions for 1802 

 and 1804, unexpected phenomena were communicated. Instances 

 in which two stars were performing to each other the offices of sun 

 and planet were proved to exist ; and to more than one pair the 

 period of rotation was, according to the observations of the authors 

 of this paper, ascertained with considerable exactness. Immersions 

 and emersions of stars behind each other had been witnessed ; and 

 real motions among some of them had been observed rapid enough 

 to be detected in very short intervals of time. 



But as from the novelty of the subject, and from the imperfections 

 of the micrometers employed in 1779 and 1780, it was likely that 

 some instances of error had occasionally crept in, it became desirable 

 that a second re-examination should be instituted : accordingly, in 

 the year 1816, some progress was made by Mr. Herschel towards its 

 accomplishment, and the results are communicated in the present 

 paper. A similar idea having likewise occurred to Mr. South, it was 

 at length determined that the observations should be carried on in 

 concert, and with his instruments. 



Meanwhile (unknown to the authors of this paper) a similar un- 

 dertaking had been entered upon by a distinguished continental astro- 

 nomer, Mr. Struve, Director of the Imperial Observatory at Dorpat; 

 and the general coincidence between the measures of this observer 

 and those of their own, is deemed at once interesting and corrobo- 

 rative of the accuracy of both. 



