524 BIBLIOGRAPHY OF HERSCHEL's WRITINGS. 



Herschel, W.: Synopsis of the Writings of— Coutinued. 

 A. D. Vol. P. 



1781 71 500 Description of a Micrometer for taking the angle of i^osition. (See 



Plate XXVI, figs. 1, 2, 3, 4.) 

 This is the modem form. 



1782 72 82 On the Parallax of the Fixed Stars, by Mr. Herschel, F. K. S. ; com- 



municated by Sir Joseph Banks, Bart., P. R. S. Read December 



6, 1781. 

 82 The nearest of the fixed stars caanot be less than 40,000 diameters of 



the whole annual orbit of the earth distant from us. 

 82 As we cannot enlarge this base, we can only endeavor to improve the 



instruments by which we measure the parallax. 



82 To measure small angles with accuracy two things are necessary: 1st, 



that the instrument used for the i»urpose should bo divided with 

 sufficient exactness; and 2d, that the telescope should have an ade- 

 quate power and distinctness. 



83 The first condition is (now) practically fulfilled. The chief difficulty 



is in the optical i)art. To see 1" with precision requires a telescope 

 of very great perfection. 



83 Even sui)posing the parallaxes of stars not to amount to single sec- 



onds, or even thirds [-gV']> tlie observations necessary to show this 

 would still have value. 



84 The next step necessary to consider in this undertaking was the man- 



ner of luitting it into execution. 

 84 The method jiroposed by Galilee, and attempted by Hooif, Flam- 

 steed, Molineux, and Bradley, of measuring zenith distances of 

 stai's which pass close to the zenith, though it failed with regard 

 to i^arallax, has been productive of the most noble discoveries of 

 another nature. 



84 BKvVDLEY (in Phil. Trans., No. 406, j). G37) concludes that the par- 



allax of y Draconis, or of ?/ Urna; Majoris, "is not so great as one 

 single second." 



85 y Draconis is a bright third magnitude, and the conclusion that sev- 



eral authors have reached, that the parallaxes of stars in general do 

 not exceed 1", does not appear to me to follow from the observations. 

 For aught we know to the contrary, the stars of the first magnitude 

 may still have a parallax of several seconds. 



86 The method of zenith distances labors under the following considera- 



ble difficulties: In the first iilace, the refractions; 2d, the change 

 of position of the earth's axis, arising from nutation, precession, 

 and other causes, is not completely settled ; 3d, the aberration, 

 though best known of all, may also be liable to some small errors. 



87 I shall now deliver the method I have taken and show that it is free 



from every error to which the former is liable, and is still capable 

 of every improvement the telescope and mechanism of micrometers 

 can furnish. 

 87 Let O and E (fig. 1) be two opposite points of the earth's orbit, in the 

 same plane with two stars, a and b, of unequal magnitude. Let the 

 angle aOb be observed when the earth is at O, and aE& when the 

 earth is at E. From the difference of these angles we may calculate 

 the parallax of the stars. These two stars ought to be as near each 

 other as possible, and also difl'er as much in magnitude as we can 

 find them. 



