Not without failures and disheartening complications was this part of the process completed.  This was, after all, the real tug of war.  Many unknown quantities existed:  its mass, its distance, its eccentricity, the obliquity of its orbit, its position—­nothing was known, in fact, about the planet except the microscopic disturbance it caused in Uranus, several thousand million miles away from it.  Without going into further detail, suffice it to say that in June, 1846, he published his last paper, and in it announced to the world his theory as to the situation of the planet.

Professor Airy received a copy of this paper before the end of the month, and was astonished to find that Leverrier’s theoretical place for the planet was within 1 deg. of the place Adams had assigned to it eight months before.  So striking a coincidence seemed sufficient to justify a Herschelian sweep for a week or two.  But a sweep for so distant a planet would be no easy matter.  When seen through a large telescope it would still only look like a star, and it would require considerable labor and watching to sift it out from the other stars surrounding it.  We know that Uranus had been seen twenty times, and thought to be a star, before its true nature was discovered by Herschel; and Uranus is only about half as far away as Neptune.

Neither at Paris nor at Greenwich was any optical search undertaken; but Professor Airy wrote to ask M. Leverrier the same old question that he had fruitlessly put to Adams:  Did the new theory explain the errors of the radius vector or not?  The reply of Leverrier was both prompt and satisfactory—­these errors were explained, as well as all the others.  The existence of the object was then for the first time officially believed in.  The British Association met that year at Southampton, and Sir John Herschel was one of its sectional presidents.  In his inaugural address, on September 10, 1846, he called attention to the researches of Leverrier and Adams in these memorable words: 

“The past year has given to us the new [minor] planet Astraea; it has done more—­it has given us the probable prospect of another.  We see it as Columbus saw America from the shores of Spain.  Its movements have been felt trembling along the far-reaching line of our analysis with a certainty hardly inferior to ocular demonstration.”

It was nearly time to begin to look for it.  So the astronomer-royal thought on reading Leverrier’s paper.  But as the national telescope at Greenwich was otherwise occupied, he wrote to Professor Challis, at Cambridge, to know whether he would permit a search to be made for it with the Northumberland equatorial, the large telescope at Cambridge University, presented to it by one of the Dukes of Northumberland.

Professor Challis said he would conduct the search himself, and shortly began a leisurely and dignified series of sweeps around the place designated by theory, cataloguing all the stars he observed, intending afterward to sort out his observations, compare one with another, and find out whether any one star had changed its position; because if it had it must be the planet.  Thus, without giving an excessive time to the business, he accumulated a host of observations.