HISTORY OF PHYSICAL ASTRONOMY.
IDS
whether they had secured the discovery of the planet. Having resumedthe comparison of the observations of the 30th July and the 12th August,which both related to the same zone, he found that a star, marked No. 49in the series' of the 12th August, was wanting in the series of the 30thJuly. It followed, as a necessary consequence, that this was the planet:it had wandered into the zone during the period that elapsed between thetwo observations. He also easily ascertained, by means of the observationof the 12tli August, that the planet was included in the stars observed asreference points on the 4tli of the same month*. Thus, although the 12thAugust was only the fourth day of observing, two positions of the planetwere already secured. “ This is entirely to be attributed,” says ProfessorCliallis, “ to my having on those days directed the telescope towards theplanet’s theoretical place, according to instructions given in a paper Mr.Adams had the kindness to draw up for me.” j-
It is a remarkable fact tlipt before receiving intelligence of the discoveryof the planet by Galle, Professor Challis had also obtained a position ofthe planet by pursuing the plan of observation recommended by Le Verrier .On the 20th September he received the Com])tes Rendus for the 31stAugust, containing the account of that geometer’s researches on thehypothetic planet. Struck with the author’s conclusions relative to thelimits of the planet’s position and the magnitude of its disk, he resolvedto attempt the discovery of the body by means of its physical appearance.He possessed an advantage in prosecuting a search of this kind which fewastronomers enjoyed in an equal degree, from having at his command themagnificent equatorial belonging to the Observatory . On the evening ofthe 29th September he examined a zone comprised between the limits ofright]ascension, within which Le Verrier had fixed the position of theplanet. Among 300 stars which passed through the field of view of histelescope, one especially attracted his attention by its disk. This provedto be the planet; it shone with the lustre of a star of the eighth magnitude.
The search commenced by Professor Challis on the 29th July, andprosecuted with so much energy and perseverance during the two following months, is deserving of attention, both on account of its forming theonly systematic attempt to detect the planet that had been made previousto the evening of its actual discovery, as well as on account of the resultswhich were obtained by the subsequent discussion of the observations. Itaj)pears that not only was the theoretical discovery of the planet first effectedat Cambridge, but two positions of it were also secured at the same placesix weeks before a telescope was directed to the heavens in search of it atany other observatory in Europe . In estimating how nearly ProfessorChallis had arrived at the actual discovery of the planet, it must bo bornein mind that the contemplated search for it extended only to a definiteregion, of the heavens, and that it would have been completed within adefinite lapse of time. When the observations came to be discussed, thediscovery of the planet would infallibly have resulted from a comparisonof the observation of the 30th July with that of the 12tli August.
The earliest announcement of Mr. Adams’ researches through the
* The following are the positions of the planet in right ascension and declination, asassigned by these observations :—
M.T. Greenwich. Right Ascension. North Polar distance.
August 4 . . 13 h 26 m 25 s .. 21 h 58 m 14’.70 .. 102 J 57' 32".2
„ 12 . . 13 3 20 .. 21 57 20.13 . . 103 2 0 .2
O