It was the 23rd September, 1846, when the request from Le Verrier reached the Berlin Observatory, and the night was clear, so that the memorable search was made on the same evening.  The investigation was facilitated by the circumstance that a diligent observer had recently compiled elaborate star maps for certain tracts of the heavens lying in a sufficiently wide zone on both sides of the equator.  These maps were as yet only partially complete, but it happened that Hora.  XXI., which included the very spot which Le Verrier’s results referred to, had been just issued.  Dr. Galle had thus before his, eyes a chart of all the stars which were visible in that part of the heavens at the time when the map was made.  The advantage of such an assistance to the search could hardly be over-estimated.  It at once gave the astronomer another method of recognising the planet besides that afforded by its possible possession of a disc.  For as the planet was a moving body, it would not have been in the same place relatively to the stars at the time when the map was constructed, as it occupied some years later when the search was being made.  If the body should be situated in the spot which Le Verrier’s calculations indicated in the autumn of 1846, then it might be regarded as certain that it would not be found in that same place on a map drawn some years previously.

The search to be undertaken consisted in a comparison made point by point between the bodies shown on the map, and those stars in the sky which Dr. Galle’s telescope revealed.  In the course of this comparison it presently appeared that a star-like object of the eighth magnitude, which was quite a conspicuous body in the telescope, was not represented in the map.  This at once attracted the earnest attention of the astronomer, and raised his hopes that here was indeed the planet.  Nor were these hopes destined to be disappointed.  It could not be supposed that a star of the eighth magnitude would have been overlooked in the preparation of a chart whereon stars of many lower degrees of brightness were set down.  One other supposition was of course conceivable.  It might have been that this suspicious object belonged to the class of variables, for there are many such stars whose brightness fluctuates, and if it had happened that the map was constructed at a time when the star in question had but feeble brilliance, it might have escaped notice.  It is also well known that sometimes new stars suddenly develop, so that the possibility that what Dr. Galle saw should have been a variable star or should have been a totally new star had to be provided against.