of Edinburgh, Session 1880-81. 
89 
3. Additional Note on an Ultra-Neptunian Planet. 
By Professor George Forbes. 
On the 16tli of February 1880 I communicated to the Society a 
research upon Comets, which led me to believe in the existence of 
two new planets outside the orbit of Neptune. This memoir was 
not published in the “ Transactions,” and only a short abstract 
appeared in the Proceedings of the Society. A small number of 
copies were privately printed ; but any one wishing for information 
on the subject will find it most readily in the “ Observatory,” 1880, 
June 1. 
The decided tendency of the aphelia of comets to group them- 
selves at distances from the sun equal to those of the planets could 
leave no doubt about the existence of these two new planets. The 
only doubtful point was as to the positions of the planets in their 
orbits. Meantime this research gives us greater confidence in the 
accuracy of the orbits of comets which have been calculated. This 
is most especially the case with the periods of revolution of comets 
when those periods are long, an element which has hitherto been 
very generally mistrusted. The most remarkable case is that of 
the comet 1861, which was calculated by Oppolzer to have a 
period of 415 years, and which I showed to have returned three 
times at such an interval. 
It is well here to remark that, even in those cases where planetary 
perturbations may have occurred, it is seldom that the omission to 
take account of these could give rise to serious error, since a 
mistake of ten years in the date of reaching the aphelion would 
affect the longitude of the planet only to a trifling extent. 
At present I have to speak only of the nearest of the two new 
planets. It was shown in the previous research to lie, in the year 
1880, either close to the ecliptic in longitude 174°, or else on an 
orbit inclined to the ecliptic at 53°, with the longitude of the 
ascending node at 253°, the longitude of the planet on this orbit 
being 185°. The coincidences in the times of the comets arriving 
at their aphelia, and the times of a hypothetical planet reaching 
those positions, were so striking as to leave little doubt about one 
of these two being the true position. 
