HISTORY OF PHYSICAL ASTRONOMY.

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sensible disc. The apparent diameter, however, which he assigned to itwas so extremely small that it was manifest only the most powerful tele-scopes would suffice to distinguish it from the fictitious discs exhibited bythe fixed stars. The probability, therefore, of discovering the planet byits physical appearance was confined to a limited number of European ;; Observatories . With respect to the method of discovering the planet by

j its proper motion, the use of the telescope in this case is mainly to render

the star visible as a luminous point, and, in consequence, it is more gene-rally practicable than the method just referred to. On the other hand,the operation of carrying it into effect is extremely laborious, unless the, astronomer already possesses a map of the region of the heavens which

i he purposes to examine, including all the stars down to the magnitude of

! the body he is in search of. With such a guide, however, nothing can be

j more simple than to ascertain whether the region which he is engaged in

j exploring can possibly afford any indication of the planet. For this pur-

j pose the astronomer has only to compare the actual appearance of the

j heavens on any night with the map. If the stars in both cases correspond,

; it follows that no change has occurred since the construction of the map,

I and, as all the objects whose positions were recorded must in consequence

h have been of a stellar nature, the comparison of the heavens with themap cannot afford any clue to the existence of a planet. If, however, themap contains a star which is not in the heavens, it is clear that the miss-ing star must have been a planet which wandered out of the region underexamination during the period that elapsed since the construction of themap, and its discovery may be expected to result from a careful scrutiny ofthe heavens in the vicinity of the stars designated on the map. On theother hand, if a star appear in the heavens which is not contained in themap, it clearly indicates the entrance of a planet into the designated re-gion subsequently to the construction of the map. In order to conduct asearch for the Trans-Uranian planet after this manner, it was necessary topossess a map on which were designated all the stars in the part of theheavens assigned for examination, down to the tenth order of magnitudeinclusive. No such map had hitherto been executed for the region compre-hending the theoretical locus of the planet, and the only method of searchwhich remained to be adopted was that already in course of being carriedinto effect by Professor Challis, and which was indeed tantamount to theactual construction of a map.

On the 18th September Le Verrier addressed a letter to the astrono-mers of the Berlin Observatory, announcing to them the result of his re-searches, and requesting their co-operation in searching for the planet. Bya singular instance of good fortune the Berlin astronomers possessed anadvantage in effecting this search which was not yet available to the otherastronomers of Europe . For some years past a series of star maps had' been in course of publication, under the auspices of the Berlin Academy of Sciences , comprehending different portions of the region of the heavenswhich extends 15° on each side of the equator, and designating the posi-tions of all stars down to the tenth magnitude*. Just as the accounts re-specting Le Venders researches reached Berlin, the map of Hora XXI.,the part of the heavens containing the theoretical place of the planet—which had been executed with great care by Dr. Bremiker —was engraved

* Many astronomers in other countries of Europe , as well as Germany , have lent theiraid in the construction of these maps. One of them was executed by Dr. Hussey, theastronomer to whom allusion has been made at the beginning of this chapter.