216 Olservalions for delennining 



In the paperwhich I had the honour of presenting to the Roy.l! 

 Society last year, I mentioned my doubts as to the quantity of 

 the maximum ot tiie aberration of light ; and that, as far as could 

 be ascertained from Dr. Bradley's Wanstead observations with a 

 zenith sector, we ought rather to adopt 20",U() than 2U",2."). 

 I also mentioned that it would he desirable to investigate this 

 point, and therefore during the last year, I instituted a course of 

 observations for this purpose, and I beg leave to offer the results 

 thereof. 



By these the maximum appears to be 20'',S0, which is much 

 greater than I had expected. While these observations were 

 going forward, Mr. Bessel's work above mentioned was pub- 

 lished. From several investigations in the Greenwich observa- 

 tions of Dr. Bradley, he also deduced the maximum = 20",70, 

 nearly. These results certainly appear extraordinary, and are 

 not likely to be acknowledged by astronomers, unless they shall 

 be established l)y a great number of observations. 



My results were computed with great care, allowances being 

 made for the ellipticity of the eartii's orbit. It is not likely, 

 supposing the velocity of the light of all the stars to be the same, 

 that the result can err more than i of a second*. 



By continuing the observations, I hope to obtain further in- 

 formation on this interesting point. And it appears to be an 

 inquiry deserving of the joint co-operation of astronomers. 



* The observations of Mr. Pond with the fixed telescope, maybe adduced 

 as contrary to my results; because with this maxinuim of aberration, his 

 summer and winter differences of N. P. distance of /3 Aurigae and a Cygni 

 would differ by 1" in a direction contrary to parallax. But it also seems to 

 show the necessity of exact determination of the precise quantities of the 

 ♦'(j/iations for N. P. D. before any conclusive arguments respecting the nor- 

 existcnce of parallax, from observations of the positions of stars relative to 

 each other can be adduced. In observations by the eight feet circle this i* 

 BOt so necessarv, as has been before mentioned. 



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