536 Prof. G. B. Airy on the History of the Discovery of 



detected it. On account of moonlight I did not observe again 

 till August 12. On that day I went over again the zone of 

 9' breadth which I examined on July 30. * * * The space 

 gone over on August 12, exceeded in length that of July 30, 

 but included the whole of it. On comparing [at a later time] 

 the observations of these two days, I found that the zone of 

 July 30 contained every star in the corresponding portion of 

 the zone of August 12, except one star of the 8th magnitude. 

 This, according to the principle of search, which in the want 

 of a good star-map I had adopted, must have been a planet. 

 It had wandered into the latter zone in the interval between 

 July 30 and August 12. By this statement you will see, that, 

 after four days of observing, the planet was in my grasp, if 

 only I had examined or mapped the observations. I delayed 

 doing this, partly because I thought the probability of disco- 

 very was small till a much larger portion of the heavens was 

 scrutinized, but chiefly because I was making a grand effort 

 to reduce the vast number of comet observations which I have 

 accumulated ; and this occupied the whole of my time when I 

 was not engaged in observing. I actually compared to a 

 certain extent the observations of July 30 and August 12, soon 

 after taking them, more for the sake of testing the two methods 

 of observing adopted on those days than for any other purpose ; 

 and I stopped short within a very few stars of the planet. After 

 August 12, I continued my observations with great diligence, 

 recording the positions of, I believe, some thousands of stars : 

 but I did not again fall in with the planet, as 1 took positions 

 too early in right ascension. * * * On Sept. 29, however, 

 I saw, for the first time, Le Verrier's last results, and on the 

 evening of that day I observed strictly according to his sug- 

 gestions, and within the limits he recommended ; and I was 

 also on the look-out for a disc. Among 300 stars which I 

 took that night, I singled out one, against which I directed my 

 assistant to note " seems to have a disc," which proved to be 

 the planet. I used on this, as on all other occasions, a power 

 of 160. This was the third time I obtained an approximate 

 place of the planet before I heard of its discovery." 



This letter was written to me purely as a private communi- 

 cation, but I have received permission from Professor Challis 

 to publish it with the rest. Before terminating this account, 

 I beg leave to present the following remarks : — 



First. It would not be just to institute a comparison between 

 papers which at this time exist only in manuscript, and papers 

 which have been printed by their authors; the latter being in 

 all cases more complete and more elaborately worked out than 

 the former. 



