Hi PROFESSOR CHALLIS'S REPORT 



In my answer, dated July 18, I signified the determination I had come to of undertaking 

 the search. Various reasons led me to this conclusion. I had already, as Mr Adams can 

 testify, entertained the idea of making these observations; the most convenient time for 

 commencing them was now approaching; and the confirmation of Mr Adams's theoretical 

 position by the calculations of M. Le Verrier appeared to add very greatly to the pro- 

 bability of success. I had no answer to make to Mr Airy's offer of sending an assistant, 

 as I understood the acceptance of it to imply the relinquishing on my part of the 

 undertaking. 



I have now to speak of the observations. The plan of operations was formed mainly 

 on the suggestions contained in Mr Airy's note of July 13. It was recommended to 

 sweep over, three times at least, a zodiacal belt 30 long and 10 broad, having the 

 theoretical place of the planet at its centre; to complete one sweep before commencing 

 the next; and to map the positions of the stars. The three sweeps, it was calculated, 

 would take 300 hours of observing. This extent of work, which will serve to show the 

 idea entertained of the difficulty of the undertaking before the planet was discovered, 

 did not appear to me greater than the case required. It will be seen that the plan did 

 not contemplate the use of hour XXI. of the Berlin Star Maps, the publication of which 

 was equally unknown at that time to Mr Airy and myself. It may be proper here to 

 explain that the construction of a good star-map requires a great amount of time and 

 labour both in observing and calculating, and that precisely this sort of labour must be 

 gone through to conduct a search of the kind I had undertaken. The stars must first 

 be mapped before the search can properly be said to begin. With a map ready made, the 

 detection of a moving body, as it happened in this instance, might be effected on a com- 

 parison of the heavens with the map by mere inspection. Not having the advantage of 

 such a map, I proceeded as follows. I noted down very approximately the positions of 

 all the stars to the llth magnitude that could be conveniently taken as they passed 

 through the field of view of the telescope, the breadth of the field with a magnifying 

 power of 166 being 9', arid the telescope being in a fixed position. When the stars came 

 thickly, some were necessarily allowed to pass without recording their places. Wishing to 

 include all stars of the llth magnitude, I proposed, in going over the same region a 

 second time, to avail myself of an arrangement peculiar to the Northumberland Equatorial, 

 the merit of inventing which is due to Mr Airy. The Hour-circle, Telescope, and Polar 

 Frame are movable by clockwork, which may be regulated to sidereal time nearly. 

 While this motion is going on, the Telescope and Polar Frame are movable relatively 

 to the Hour-circle, by a tangent-screw apparatus, and a handle extending to the observer's 

 seat. This contrivance enables the observer to measure at his leisure differences of Right 

 Ascension however small, and therefore meets the case of stars coming in groups. The 

 observations made by this method might include all the stars it was thought desirable to 

 take, and therefore might include all the stars taken in the first sweep. The discovery 

 of the planet would result from finding that any star in the first sweep was not in its 

 position in the second sweep. If two sweeps failed in detecting the planet among the 

 stars of the first sweep, it might be among the stars of the second, which would be 

 decided by taking a third sweep of the same kind as the second. It will appear that 

 this plan carried out would not only detect the planet if it were in the region explored, 

 but would also, in case of failure, enable the observer to pronounce that it was not in 



