524 'BIBLIOGRAPHY OF HEESCHEL's WRITINGS. 



Herscliel, V/.: Synopsis of the Weitixgs of— Continued, 



A. D. Yol. p. 



1781 71 500 Desci'iption of a Micrometer for taking the angle of jDosition. (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. R. S. ; com- 



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



6, 1781. 

 82 The nearest of the fixed stars cannot 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 instrumeut used for the purpose should be divided with 

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

 quate power and distinctness. 



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



is in the optical part. To see 1" with precision requires a telescope 

 of very great perfection. 



83 Even supposing the pai'allaxes of stars not to amount to single sec- 



onds, or even thirds l^"'\, the observations necessary to show this 

 would still have value. 



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



ner of putting it into execution. 

 84 The method i)roposed by Galilee, and attempted by Hook, Flam- 

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

 stars which pass close to the zenith, though it failed with regard 

 to parallax, has been productive of the most noble discoveries of 

 another nature. 



84 Br^vdley (in Phil. Trans., No. 406, p. 637) concludes that the par- 



allax of;' Draconis, or of 7; Ursw 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 jiarallaxes of stars in general do 

 not exceed \", 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 place, 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 h, of unequal magnitude. Let the 

 angle aOh be observed when the earth is at O, and aEZ* when the 

 earth is at E. From the diflference 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 differ as much in magnitude as we can 

 find them. 



