the Planet exterior to Uranus. 535 



found that Venus had phases like the Moon, nothing (in my 

 opinion) so bold, and so justifiably bold, has been uttered in 

 astronomical prediction. It is here, if I mistake not, that we 

 see a character far superior to that of the able, or enterprising, 

 or industrious mathematician; it is here that we see the philo- 

 sopher. The mathematical investigations will doubtless be 

 published in detail ; and they will, as mathematical studies, be 

 highly instructive: but no details published after the planet's 

 discovery can ever have for me the charm which I have found 

 in this abstract which preceded the discovery. 



I understand that M. Le Verrier communicated his prin- 

 cipal conclusions to the astronomers of the Berlin Observatory 

 on September 23, and that, guided by them, and comparing 

 their observations with a star- map, they found the planet on 

 the same evening. And I am warranted by the verbal assu- 

 rances of Professor Challis in stating that, having received the 

 paper on September 29, he was so much impressed with the 

 sagacity and clearness of M. Le Verrier' s limitations of the 

 field of observation, that he instantly changed his plan of ob- 

 serving, and noted the planet, as an object having a visible 

 disc, on the evening of the same day. 



My account, as a documentary history, supported by letters 

 written during the events, is properly terminated : but I think 

 it advisable, tor the sake of clearness, to annex extracts from 

 a letter which I have received from Professor Challis since 

 the beginning of October, when I returned to England. 



No. 23. Professor Challis to G. B. Airy. 

 [extract.] 

 "Cambridge Observatory, October 12, 1846. 

 "I had heard of the discovery [of the new planet] on Oc- 

 tober 1. * * * I find that my observations would have 

 shown me the planet in the early part of August, if I had only 

 discussed them. I commenced observing on July 29, attacking 

 first of all, as it was prudent to do, the position which Mr. 

 Adams's calculations assigned as the most probable place of 

 the planet. On July 30, I adopted the method of observing 

 which I spoke of to you * * * In this way I took all 

 the stars to the 11th magnitude in a zone of 9' in breadth, and 

 was sure that none brighter than the 11th escaped me. My 

 next observations were on August 4. On this day * * * 

 I took stars here and there in a zone of about 70' in breadth, 

 purposely selecting the brighter, as I intended to make them 

 reference points for the observations in zones of 9' breadth. 

 Among these stars was the planet. A comparison of this day's 

 observations with a good star-map would most probably have 



