Page:History of the Royal Astronomical Society (1923).djvu/107

 1840-50] ROYAL ASTRONOMICAL SOCIETY 89 English claim are mixed, it is not to be regretted that the Society was debarred by the operation of its own laws from deciding between them, of which more hereafter. Parallax : Bessel and Henderson. This careful balance is well exemplified in the case of the Medal awarded to Bessel for the first unquestionable determination of a stellar parallax. This was in 1841. Astronomy had long been plagued with will-o'-the- wisp parallaxes. The annual variation that was looked for opened possibilities of confusion with seasonal changes which might be atmospheric or of various other kinds. The meridian methods employed were not well adapted either for absolute or the highest class of differential determination. Unless a clear, confirmed progression could be shown month by month through the cycle, which could arise from no other cause, suspense and even scepticism was the proper attitude. It has already been mentioned above that Henderson had returned from the Cape in 1833, bringing with him his observations for reduction. He aimed to be, and was, as thorough and careful of instrumental details as Bessel himself, and his discussion of the removal of errors from readings of the Cape mural circle was accepted as a model. In 1839 he produced his discussion of observations of a Centauri. The declination, subjected to every test that he could put, agreed with a parallax of about I ". Yet by common consent, perhaps not excluding Henderson's own, the matter was held as not proven, until Maclear, his successor, should produce a further series that would confirm it. The amount was felt to be large. We now know that the parallax is large, the modern accepted value is o"-y6 ; it is the nearest lucid star yet found ; but Struve had shown, twenty years earlier, that not one out of 27 circumpolar stars whose right ascensions he had examined possessed a parallax of half a second. The confirmation was forthcoming in 1842 ; but it was not reassuring that twenty other stars in Maclear's list showed an average prima facie parallax of o"-3. Not one of these has been confirmed. Henderson remarks : "In a conversation I had with M. Bessel," whose friendship was his boast and delight, and whom he consciously took as his model in matters astro- nomical, " he expressed his wish that a Centauri were observed with a heliometer, or good equatoreal, capable of precise micro- metrical measurement ; he said he had doubts of the results derived from meridian instruments. He mentioned the case of Dr. Brinkley's parallaxes, and stated that in his own observatory two excellent meridian circles, placed beside each other, gave at certain seasons places of the pole star that differed from each other ; the reason of which disagreement he had not found out." On the other hand, Bessel's own heliometer measures of 61 Cygni left no