226 D. P. Todd—Search for a Trans-neptunian Planet. 
observed orbit as to indicate that his computations did not per- 
tain to the actual disturbing planet, elicited from him the reply 
that the perturbations of Uranus due to a possible planet exte- 
rior to Neptune might readily cause an uncertainty of 5’-to 7” 
in the fundamental data of his researc 
In 1866, the Smithsonian Institution published the general 
tables of Neptune, by Professor Newcomb. In the investiga- 
tion of its orbit the author proposed: “3. To i xiao whether 
those motions [of Neptune] indicate _ action of an extra- 
Neptunian planet, or throw any light on the question of the 
existence of such a planet.” He iemais (page 73) that it is 
“almost vain to hope for the detection of an extra-Neptunian 
planet from the motions of Neptune before the close of the 
present centur 
In 1873, the Smithsonian cae Sra the general 
tables of Uranus, is Siegen weomb. His success in the 
b 
known, ae me that since the ibeaticn of these tables the 
error of longitude has been on the negative increase, and the 
latest observations place the planet increasingly more in ad- 
vance of its theoretic position. 
Sometime in the spring of 1874, the first preliminary outline 
of the very simple method which 'T have here employed in the 
treatment of planetary residuals with reference to exterior per- 
turbation, suggested itself to me. For more than three years, 
very little opportunity offered for consideration of the problem 
of a trans-neptunian planet, and I gave it merely desultory 
attention. In August, 1877, however, 1 began to devote the 
arger portion of my leisure time to the theoretic side of the 
question. It was soon evident that no certain hold upon any 
possible cause of exterior pres eo could be obtained from 
the residuals of Newcomb’s tables. An may remark here 
that I have consequently éhowen the term speculative rather 
than theoretic as applying more fitly to the investigation which 
preceded the actu 5 ialioicess search. 
The Speculative Search. 
While the magnificent researches of LeVerrier and Adams 
on the perturbations of Uranus are masterpieces of analytic 
skill, I felt that they should a be taken as models in the 
present investigation—for two 
(1) The residuals of ibaitude which must form the basis of 
