remark which led immediately to its discovery. Until this was so decisively stated, it must have seemed exceptionally improbable; for we saw in the last chapter how diligently the Zodiac had been swept in the search for minor planets,--how, for instance, Hencke had searched for fifteen years without success; and it might fairly be considered that if there were a fairly bright object (such as Neptune has since been found to be) it would have been discovered earlier. Hence Airy not unreasonably considered it necessary to spread his net for very small objects. On July 9 he wrote to Professor Challis as follows:-- No. 15.--G. B. AIRY _to_ PROFESSOR CHALLIS. "THE DEANERY, ELY, _1846, July 9_. "You know that I attach importance to the examination of that part of the heavens in which there is ... reason for suspecting the existence of a planet exterior to _Uranus_. I have thought about the way of making such examination, but I am convinced that (for various reasons, of declination, latitude of place, feebleness of light, and regularity of superintendence) there is no prospect whatever of its being made with any chance of success, except with the Northumberland telescope. "Now, I should be glad to ask you, in the first place, whether you could make such an examination? "Presuming that your answer would be in the negative, I would ask, secondly, whether, supposing that an assistant were supplied to you for this purpose, you would superintend the examination? "You will readily perceive that all this is in a most unformed state at present, and that I am asking these questions almost at a venture, in the hope of rescuing the matter from a state which is, without the assistance that you and your instruments can give, almost desperate. Therefore I should be glad to have your answer, not only responding simply to my questions, but also entering into any other considerations which you think likely to bear on the matter. "The time for the said examination is approaching near." [Sidenote: Challis undertakes the search.] [Sidenote: He finds too late that he had observed the planet.]