COMMENTATIO ASTRONOMICA. 91 



S 9. 



Anno 1781 Herschelius paler distantias duplicium observandas proi)Osuit , edita 

 fcriplione (*) , cujus summa, quod quidem ad methodum ipsam attinct , huc redit : 



« Let OE be two opposite poinls of the annual orbit , laken in the same pla- 

 -« ne wilh two stars a, b, of unequal magnitudes. Let the angle «05 be observed 

 « wben the earlh is at , and let the angle aE6 be also observed when the earlh is 

 <( at E. From the diJöference of these angles , if any should be found , we may calcu- 

 « late the parallax of the stars , according to a theory that will be delivered hereafler. 

 « These two stars , for reasons that will soon appear , ought to be as near each olher 

 STas possible, and also to differ as much in magnitude as we can find them. " 



Ergo stellae duplices subtiles- intra 5" distantes accurate, quoad situm relativum , ob- 

 servantor. 



« Bul to proceed , I shall now proTe that this method , if stars properly situated 

 « (such as I have found) are taken, is free from all the errors occasioned by refrac- 

 « lion, nutation, precession of the equinoxes, changes of the obliquily of the ecliptic , 

 « and aberration of light; and that the 'annual parallax, if it even should not exceed 

 « the tenth part of a second , may still become visible , and be ascertained at least to 

 « a much greater degree of approximation than it ever has been done. " 



« It will also appear, from the great number of observalions I have already made 

 « upou several double stars, especially s Boolis, that we can now with much greater 

 « certainty affirm the annual parallax to be exceedingly small indeed ; and that there is 

 « a great probability of succeeding still farther in this laborious but delightful research , 

 «t so as to be able at last to say, not only how much the amiual parallax is wt , but 

 n how much it really is. " 



Demonstrat autem harum vicinaram situm relativum non mutari refraofione, praecessio- 

 ne, nutatione, aberratione. 



« Observations of the relative distance of Ihe two stars that make up a double Star, 

 « being thus cleared of evcry impediment, are capable of being continually improved by 

 « every degree of perfection the lelescope may acquire : we can chuse stars that may be 

 « viewed sufficiently high to be clear of the vapours that swim near the horizon , and 

 « consequently employ the greatest powers our Instruments are capable of." 



Commendat vim opticam ingenlem, cujus experimentis fuerat duclus ad inveniendum 



(•) Phil. Tranu 1782. LXXIt, P. I. p. 83. « On the parallas of the fixed stars, ty Mr. Her- 

 , tchel, F. ß P," 



M 2 



