Y2 Royal Astronomical Society. 



and could be used for comparison. The investigation, like all others 

 which he undertook, is conducted in the most careful manner ; and 

 the result, although, in consequence of the mode of proceeding 

 adopted, it is dependent on the accuracy of the determination of the 

 relative declinations of the principal stars, is probably the best de- 

 termination of the constant we are in possession of. 



It would almost seem as if, in these investigations, Mr. Hender- 

 son had it in view to repeat the labours of Lacaille in his memorable 

 residence at the Cape about the middle of the last century. In an- 

 other and more important feature he may be considered as an imi- 

 tator of that great astronomer. Lacaille had remarked that, in the 

 then advanced state of astronomy no one could any longer be believed 

 on his mere word ; and that, in order to employ with confidence an 

 observed position, it was necessary to have all the details of the ob- 

 servation and all the elements of reduction. No astronomer was 

 ever more careful of the observance of this important maxim than 

 Mr. Henderson. From the reading of the circle to the final result 

 of the investigation, every thing necessary for the complete under- 

 standing of the process or verification of the work is carefully set 

 forth and explained ; and, accordingly, all his investigations and re- 

 sults are characterized by an air of truthfulness which it would be in 

 vain to look for where a different practice is followed. 



But of all the results which he deduced from this persevering ex- 

 amination of his Cape observations, the one which will be considered 

 as the most interesting is the annual parallax of the double star a 

 Centauri, amounting to about a second of space. This binary sy- 

 stem was recognised to be double about a century ago ; and, on 

 comparing the observations of Lacaille with those of the present 

 time, it is found to have an annual proper motion of about 3"' 6. 

 Being situated within about 30° of the South Pole, it is always 

 above the horizon at the Cape, and consequently favourably placed 

 for being accurately observed at all times. The intrinsic brightness 

 of the two component stars is also remarkable ; and from this, and 

 their large proper motion — circumstances indicative of proximity to 

 our system — Mr. Henderson was led to suspect they might have a 

 sensible parallax. He was not, however, aware of their large proper 

 motion till he was about to leave the Cape ; and the observations 

 from which the result was first deduced, were accordingly not made 

 with especial reference to the question of parallax, but for the pur- 

 pose of determining their mean positions. The first indications of 

 parallax were detected upon a comparison of their declinations with 

 those of such of the standard stars as were observed on the same day 

 throughout the year ; but he deferred the announcement of the dis- 

 covery until he had com])leted the reduction of the right ascensions, 

 and obtained, by this means, a further test of the accuracy of his 

 conclusion. A similar investigation of these latter observations con- 

 firmed the previous deduction ; and, in a paper read to the Society 

 at the January meeting of 1839, he announced a parallax of the 

 double star amounting to about a second of space. With his habi- 

 tual caution and accuracy, he applied every means of testing the re- 



