THE ORBIT OF NEPTUNE. 61 



REMARKS ON THE PRECEDING TABLE. 



The processes to which we have subjected the observations ought, it would 

 seem, to eliminate every source of constant differences between those made at 

 different observatories. But there are still two well-marked cases of systematic 

 differences in the right ascensions, namely, in the Cambridge observations of the 

 first five years, and the Albany observations of the last four. The differences 

 between the corrections finally concluded from all the observations, and those 

 concluded from Cambridge and Albany, are, it will be seen, as follows : 



Date. Cone. Camb. Date. Cone. Albany. 



S S 



1846, Oct. + 0.02 1861, Sept. 0.09 

 Nov. +0.06 Oct. 0.06 



1847, July, +0.02 Dec. 0.05 

 Aug. + 0.03 1862, Aug. 0.06 

 Oct. +0.08 Sept. -0.08 

 Nov. +0.07 Nov. 0.07 



1848, July, + 0.07 Dec. 0.09 

 Aug. + 0.07 1863, Sept. 0.06 

 Oct. +0.01 Nov. 0.04 

 Nov. +0.07 Dec. 0.01 



1849, Sept. + 0.06 1864, Oct. - 0.05 

 Oct. +0.10 Nov. -0.08 

 Nov. 0.03 Dec. 0.03 



1850, Aug. + 0.04 

 Oct. + 0.05 

 Nov. + 0.01 



The constancy of signs here exhibited can hardly be attributed to chance in 

 the case of Cambridge, and not at all in the case of Albany. The only cause 

 to which I can attribute it is a habit of registering the transit of Neptune earlier 

 or later than that of a bright star. Such a habit would seem to pertain to the 

 observer rather than the instrument, and, therefore, less to be feared as the number 

 of observers is increased. On account of its possible existence, the weights of the 

 results of any one observatory have not been supposed proportional to the number 

 of observations, but each has been subject to a constant probable error of at least 

 0'.02 when observations were made by eye and ear, and O s .01 when made with 

 chronograph, however great the number of observations. 



Albany exhibits the anomaly that the real systematic error seems greater than 

 the probable accidental error. The latter is of the smallest class, as might be 

 anticipated from the facts that the observations are made with a first-class in- 

 strument, in a good atmosphere, and are recorded with the electro-chronograph. 

 They have, therefore, been treated in such a way that, while they should enter 

 the absolute longitudes with a very small weight, they should enter the relative 

 longitudes at different times of the year, in other words, the radius vector, with 



