516 Dr, Bobert S. Ball [Feb. 11, 



comparison stars the two objects marked with his name. He 

 measured the distance from the central point of 61 Cygni to each of 

 the two comparison stars. From a series of such measures he dis- 

 covered the parallactic ellipse of 61 Cygni. He was led to the same 

 ellipse by each of the two comparison stars. 



Fifteen years latter (1853) Struve undertook a new determination. 

 He chose a comparison star different from either of those Bessel had 

 used. Struve's method of observing was also quite different from 

 Bessel's. Struve made a series of measures of the distance and 

 position of the comparison star from 61 (B) Cygni. Struve succeeded 

 also in measuring the parallactic ellipse. 



There was, however, an important difference between their results. 

 The distance, according to Bessel, was half as much again as Struve 

 found. Bessel said the distance was sixty billions of miles ; Struve 

 said it could not be more than forty billions. 



The discrepancy may be due to the comparison stars. If Bessel's 

 comparison stars were only about three times as far as 61 Cygni, 

 while Struve's star was about eight or ten times as far, the difference 

 between Struve's result and Bessel's would be accounted for. 



To settle the question, observations were subsequently made by 

 Auwers and others ; the latest of these investigations is one which has 

 recently been completed at Dunsink Observatory. 



Dr. Briinnow commenced a series of measures of the difference in 

 declination between 61 Cygni and a fourth comparison star. The 

 carrying out of this work devolved on the lecturer, as Dr. Briinnow's 

 successor. Two series of observations have been made, one with each 

 of the components of 61 Cygni. The results agree very nearly with 

 those of Struve. 



On a review of the whole question, there seems no doubt that the 

 annual parallax of 61 Cygni is nearer to the half second found by 

 Struve, than to the third of a second found by Bessel. 



To exhibit the nature of the evidence which is available for the 

 solution of such a problem, a diagram showing the observations has 

 been prepared. In the accompanying Fig. 2 the abscissae are the 

 dates of the second series of observations made at Dunsink. The 

 ordinates indicate the observed effect of parallax on the difference of 

 declinations between 61 (B) Cygni and the comparison star. Each 

 dot represents the result of the observations made on the corre- 

 sponding night. The curve indicates where the observations should 

 have been with a parallax of 0" -47. The discordances seem in many 

 cases very considerable. They are not, however, intrinsically so great 

 as might perhaps be at first thought. The distance from the top of 

 the curve to the horizontal line represents an angle of four-tenths of 

 a second. This is about the apparent diameter of a penny-piece at 

 the distance of ten miles. The discordance between the observations 

 and the curve is in no case much more than half so great. It there- 

 fore appears that the greatest error we have made in these observations 

 amounts to but two or three tenths of a second. This is equivalent to 



