TO THE OBSERVATORY SYNDICATE. li 



also corrected. As the calculations rested on an assumption, made according to Bode's 

 law, that the mean distance of the disturbing planet was double that of Uranus, without 

 the above-mentioned numerical verification, no proof was given that the problem was 

 solved or that the elements of the supposed planet were not mere speculative results. 

 The earliest evidence of the complete solution of an inverse problem of perturbations is 

 to be dated from October 1845. 



Although the comparison of the theory with observation proved synthetically that the 

 assumed mean distance was not very far from the truth, it was yet desirable to try the 

 effect of an alteration of the mean distance. Mr Adams accordingly went through the 

 same calculations as before, assuming a mean distance something less than the double of 

 that of Uranus, and obtained results which indicated a better accordance of the theory 

 with observation, and led him to the conclusion, which has since been confirmed by 

 observation, that the mean distance should be still farther diminished. This second 

 solution taken in conjunction with the first may be considered to relieve the question of 

 every kind of assumption. The new elements of the disturbing body, and the results of 

 comparing the observed with the theoretical mean longitudes of Uranus, were communi- 

 cated to the Astronomer Royal at the beginning of September 1846. These were 

 accompanied by numerical values of errors of the radius vector, the Astronomer Royal 

 having inquired, after the reception of the first solution, whether the error of radius 

 vector, known to exist from observation, was explained by this theory. It would be 

 wrong to infer that Mr Adams was not prepared to answer this question till he had gone 

 through the second solution. Errors of radius vector were as readily deducible from the 

 first solution as from the other. 



The preceding details are intended to point out the circumstances which led 

 astronomers to suspect the existence of an additional body of the solar system, and the 

 theoretical reasons there were for undertaking to search for it. No one could have 

 anticipated that the place of the unknown body was indicated with any degree of exact- 

 ness by a theory of this kind. It might reasonably be supposed, without at all mistrusting 

 the evidence which the theory gave of the existence of the planet, that its position was 

 determined but roughly, and that a search for it must necessarily be long and laborious. 

 This was the view I took, and consequently I had no thought of commencing the search 

 in 1845, the planet being considerably past opposition at the time Mr Adams completed 

 his calculations. The succeeding interval to midsummer of 1846 was a period of great 

 astronomical activity, the planet Astrasa, Biela's double comet, and several other comets, 

 successively demanding attention. During this time I had little communication with 

 Mr Adams respecting the new planet. Attention was again called to the subject by the 

 publication of M. Le Verrier's first researches in the Comptes Rendus for June 1, 1846. 

 At a meeting of the Greenwich Board of Visitors held on June 29, at which I was 

 present, Mr Airy announced that M. Le Verrier had obtained very nearly the same 

 longitude of the supposed planet as that given by Mr Adams. On July 9 I received a 

 letter from Mr Airy, in which he suggested employing the Northumberland Telescope in 

 a systematic search for the planet, offering at the same time to send an assistant from 

 Greenwich, in case I declined undertaking the observations. This letter was followed by 

 another dated July 13, containing suggestions respecting the mode of conducting the 

 observations, and an estimation of the amount of work they might be expected to require. 



