94 GERARDI REGNERI FOCKENS 



,( siuce we have 6EP = |" , and cEP = ^" , therefore iEP -. cEP = bin = |" —j" 

 ,( = J"; thnt is, the slars ö, c, will appear to be only J" removed from each other, 

 « vvlien Ihc earlh is at E. 



<f From what has Leen said, we may gallier Ihe following general expression, to de- 

 « nole Ihe parallax that will become visibie in llie change of dislance belween the two 

 h Stars , by llie removal of Ihe earth from one extreme of its orbit to the other. Let P 

 (i cxpress the total parallax of a fixed star of the first magnitude , M the magnilude of 

 « the largest of the two slars, m the magnilude of the smallest (As M and j>i are 

 « bere laken to express the relative dislances of the stars, in measures whereof the 

 <( dislance of the nearest star is taken as unity, those who tliink the postulata on 

 « which these eslimalions are built cannot be granted , may still use ^the followig 

 « formulae, if inslcad of the magnitudes M, m, they put their own eslimalions of the 

 « relative dislances of the slars, according to any other method whatever they may 

 « lliiuk it most eligible to adopt ; for the apparent magnitude of the stars is here only 

 « proposed as tlie most probable means we have of forming] any conjeclures about their 

 (c relative dislances. ) , and p the partial parallax to be observed by the change in the dis- 



j( tance of a double star ; then will p = -^ P ; and p being found by Observation 



« Will give US P = i -." 



° m — M 



Sequuntur exempla et formulae aliis adaptatae stellarum silibus. Postremo tempus et 



locus maximae parallaxeos indicantur. 



Obiter memoratur micrometrum positionis anno 1779 ab Herschelio invenlum. 



§ 10. 



Elapsis 45 annls Herscbelius filius angulos positionis slellarura duplicium astiono» 

 mis commendat, scripta coramentalioue , e qua omaem methodum in hunc locum trans» 

 tulimus ( • ) : 



« I do not find , that it has been noticed , that parallax must occasion a periodical 

 <, change in the angle of posilion , as well as in the dislance of the two stars composing 

 « a double star, and that this Variation is much more susceptible of ready and exact 

 « apprecialion wilh our present micromelers than that of their dislance. To reuder this 

 « sensible, we need only remark, that the effect of parallax is to cause each star lo 

 « describe (apparenlly) a small elUpse ia Ihe heaveiis , the major axis of which is pa- 



« ral- 



(• ) Phil, Trans. 1826. P. II. p. 266. 



