the Planet exterior to Uranus. 527 



with a request for the more accurate numbers to which he 

 alludes; but the following correspondence will show how 

 deeply his remarks had penetrated my mind. 



About a week after the receipt of M. Le Verrier's letter, 

 while on a visit to my friend, the Dean of Ely, I wrote to 

 Professor Challis as follows : — 



No. 1 5. 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, lati- 

 tude of place, feebleness of light and regularity of superin- 

 tendence) 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, whe- 

 ther 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 un- 

 formed 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 in- 

 struments 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." 



In explanation of this letter, it may be necessary to state 

 that, in common, I believe, with other astronomers at that 

 time, I thought it likely that the planet would be visible only 

 in large telescopes. I knew that the observatory of Cam- 

 bridge was at this time oppressed with work, and I thought 

 that the undertaking — a survey of such an extent as this 

 seemed likely to prove — would be entirely beyond the powers 

 of its personal establishment. Had Professor Challis assented 

 to my proposal of assistance, I was prepared immediately to 

 place at his disposal the services of an efficient assistant; and 

 for approval of such a step, and for liquidation of the expense 

 which must thus be thrown on the Royal Observatory, I 

 should have referred to a Government which I have never 

 known to be illiberal when demands for the benefit of science 



