on the presentation of a Medal. 135 



Thus it is not the absolute parallax of either, but the difference of 

 their parallaxes, which is effectively measured by this method ; i. e. 

 by repeating the measurements of their mutual distance at all times 

 of the year. But, on the other hand, aberration, nutation, precession, 

 and refraction, act equally on both stars, or so very nearly so as to 

 leave only an exceedingly small fraction of these corrections bearing 

 on the results. And when the stars are very unequal in magnitude, 

 there is a presumption that the difference of their parallaxes is very 

 nearly to the whole parallax of the nearer one. 



The selection of a star for observation involves many considera- 

 tions. In that pitched on by M. Bessel (61 Cygni), the large star 

 so designated, is in fact a fine double star ; nay, one that has been 

 ascertained to be physically double. It is in every respect a highly 

 remarkable star. The mutual distance of its individuals is great, 

 being about 16£". Now this being necessarily less than the axis of 

 their mutual orbit, affords in itself a presumption that the star is a 

 near one. And this presumption is increased by the unusually great 

 proper motion of this binary system, which amounts to nearly 5" 

 per annum, and which has been made by Sir James South the sub- 

 ject of particular inquiry, and found to be not participated in by 

 several small surrounding stars, which, therefore, are not physically 

 connected with it. Moreover, the angular rotation of the two, one 

 about the other, has been well ascertained. 



Now, it fortunately happens, that of these small surrounding stars 

 there are two very advantageously situated for micrometrical com- 

 parison with either of the individuals of ( the binary star, or with the 

 middle point between them. The one of these (a), at a distance of 

 7, 42", is situated nearly at right angles to the direction of the double 

 star ; the other (b) at a distance of 11' 46", nearly in that direction. 

 Considering (a) and (b) as fixed points then, and measuring at any 

 instant of time their distances from (c),.the middle point of the 

 double star, the situation of (c) relative to (a) and (b) is ascertained ; 

 and if this be done at every instant, the relative locus of (c), or the 

 curve described by it on the plane of the heaven with respect to the 

 fixed base-line a b, will become known. 



Now, on the hypothesis of parallax, that locus ought to be an 

 ellipse of one certain calculable eccentricity and no other. And 



