568 Royal Astronomical Society. 



that inspection certainly does leave a very strong impression of a 

 real and tolerably well-ascertained parallax having been detected in 

 this star. But at the same time an impression no less decided, 

 owing to irregularities in the march of the curve, when compared 

 with the true parallactic curve, is created, — that the errors of obser- 

 vation are far from being eliminated, — that, on the contrary, they 

 bear such a proportion to the parallax itself as to leave room for 

 some degree of hesitation, and to justify an appeal to a longer series 

 of observations, and to concurrent evidence from other quarters, 

 before declaring any positive opinion. The evidence of this kind, 

 in short, is not equal to that afforded by the similar projection of 

 Bessel's observations of either of his two comparison stars. And 

 to this it must be added, that only one star of comparison existing 

 in the line of a Lyr*, the possible effect of temperature and annual 

 instrumental variation is not eliminated from the result in tbe way 

 in which it is from the measures of 6 1 Cygni ; while all that great 

 mutual support which the observations of parallaxes of the two com- 

 parison stars afford each other in the latter case, is altogether wanting 

 in the former. These considerations, without any under-estimation 

 of the great importance and value of M. Struve's researches, yet 

 formed essential drawbacks on the immediate admission of his re- 

 sults. 



In a word, I conceive the question of discovery as between these 

 illustrious, but most generous and amicable rivals, may be thus 

 fairly stated. M. Struve's meridian observations in 1819-1821 

 seem to have made the first impression on the general problem, but 

 too slight to authorize more than a hope that it would yield at no 

 distant day. His micrometric measures of a. Lyra commenced more 

 than a year earlier, and have extended altogether over a longer pe- 

 riod than M. Bessel's of 61 Cygni . From their commencement they 

 afford indications of parallax, and these indications accumulating 

 with time have amounted to a high degree of probability, and ren- 

 dered the supposition of parallax more admissible than that of in- 

 strumental or casual errors producing the same influence on the 

 measures. On the other hand, M. Bessel's measures commencing 

 a year later, and continued on the whole through somewhat less 

 time, have exhibited a compact and consistent body of evidence 

 drawn from two distinct systems of measures mutually supporting 

 each other, and so steadily bearing on their object as to leave no 

 more reasonable doubt of its truth than in the case of many things 

 which we look upon as, humanly speaking, certain. And this con- 

 viction once obtained, reacts on our belief in the other results, and 

 induces us to receive and admit it on the evidence adduced for it ; 

 which, without such conviction so obtained, we might hesitate to 

 do until after longer corroboration of the same kind. 



The other series of observations to which I must now call your 



attention are those of Mr. Henderson, made at the Cape of Good 



Hope, on the great star a Centauri, the third star in brightness which 



the heavens offer to our view*. It is a magnificent double-star con- 



[* See Phil. Mag.,S. 3., vol. xiv. p. 316.— Ed.] 



