DISTANCES OF HEAVENLY BODIES EICHELBERGER. 175 



form of a letter from Bradley to Halley is published in the Philosophical 

 Transactions for December, 1 728 : 



When the year was completed, I began to examine and compare my obser- 

 vations, and having pretty well satisfied myself as to the general laws of the 

 phenomena, I then endeavored to find out the cause of them. I was already 

 convinced that the apparent motion of the stars was not owing to the nutation 

 of the earth's axis. The next thing that offered itself was an alteration in the 

 direction of the plumb line with which the instrument was constantly rectified ; 

 but this upon trial proved insufficient. Then I considered what refraction 

 might do, but there also nothing satisfactory occurred. At length I conjectured 

 that all the jjhenomena hitherto mentioned, proceeded from the progressive 

 motion of light and the earth's annual motion in its orbit. For I perceived 

 that, if light was propagated in time, the apparent place of a fixed object would 

 not be the same when the eye is at rest, as when it is moving in any other 

 direction than that of the line passing through the eye and the object ; and that, 

 when the eye is moving in different directions, the apparent place of the object 

 would be different. 



Wlien Bradley's observations of y Draconis were corrected for 

 aberration, they showed, according to himself, that the parallax of 

 that star conld not be as much as 1".0, or that the star was more than 

 200,000 times as distant from the earth as the smi. 



On December G, 1781, there was read before the Royal Society a 

 paper by Mr. Herschel, afterwards Sir William, on the Parallax 

 of the Fixed Stars. We read : 



The method pointed out by Galileo, and first attempted by Hook. Flamstead, • 

 Mollneaux, and Bradley, of taking distances of stars frcmi the zenith that pass 

 very near it, though it failed with regard to parallax, has been productive of 

 the most noble discovei-ies of another nature. At the same time it has given 

 us a much juster idea of the immense distance of the stars, and furnished us 

 with an approximation to the knowledge of their parallax that is much nearer 

 the truth than we ever had before * * *, 



In general, the method of zenith distances labors under the following con- 

 siderable difficulties. In the first place, all these distances, though they should 

 not exceed a few degrees, are liable to refractions ; and I hope to be pardoned 

 when I say that the real quantities of these refractions, and their differences, 

 are very far from being perfectly known. Secondly, the change of position of 

 the earth's axis arising from nutation, precession of the equinoxes, and other 

 causes, is so far from being completely settled, that it would not be very easy 

 to say what it exactly is at any given time. In the thii'd place, the aberration 

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

 since the observations from which it was deduced labored under all the 

 foregoing difficulties. I do not mean to say, that our theories of all these causes 

 of error are defective ; on the contrary, I grant that we are for most astronomical 

 purposes sufficiently furnished with excellent tables to correct our observations 

 from the above mentioned eri'ors. But when we are upon so delicate a point 

 as the parallax of the stars; when we are investigating angles that may. per- 

 haps, not amount to a single second, we must endeavor to keep clear of every 

 possibility of being involved in uncertainties ; even the hundredth part of a 

 second becomes a quantity to be taken into consideration. 



Herschel then proceeds to advocate selecting pairs of stars of very 

 unequal magnitude and whose distance apart is less than 5" and 

 making very accurate niicrometric measures of this distance from 



