370 
Proceedings of Royal Society of Edinburgh. [sess. 
Additional Note on the Ultra-Neptunian Planet, whose 
existence is indicated by its action on Comets. By 
Professor George Forbes, M.A., F.R.S. (With a Plate.) 
(Read May 6, 1901.) 
The history of research in this planet is briefly as follows : — 
In 1879 Professor Newton enunciated the proposition that if 
the elliptic orbits of comets have been changed from parabolas by 
planetary perturbations, then the probabilities are in favour of the 
comet’s position at the time becoming the aphelion position of the 
new orbit. This explains why the aphelion distances of so many 
comets agree with the mean distances of Jupiter and Neptune 
respectively. 
At the meeting of the British Association when this was 
announced, I stated that if this be true there are certainly two 
undiscovered planets beyond Neptune, one of which is at a 
distance from the sun about 100 times the mean distance of the 
earth from the sun. 
In 1880, on 16th February, I made a communication to the 
Royal Society of Edinburgh,* referring to seven comets whose 
aphelia were calculated to be at this distance, and describing an 
attempt to determine the present position of the new planet on 
the supposition that it occupied the longitudes of the several 
aphelia at dates when the comets were at those aphelion positions. 
Mr Isaac Roberts made a search by photography but did not find 
the planet, possibly owing to my having indicated for his search 
an area that was too limited. 
These calculations have lately been revised by me, use being 
made of every elliptic orbit in Galle’s recent Catalogue (Cometen- 
* A short abstract appeared in the Proceedings. I printed privately 
100 copies of the full paper, which were distributed to observatories and 
astronomers who applied for them. The present Astronomer-Royal, who at 
that time edited The Observatory , published the full paper in the issue of 
that journal for June 1880. The perturbations of Uranus by the new planet 
were discussed in a paper read to the R.S.E., 1880, May 17th. Further 
particulars were given to the R.S.E., 1881, January 17th. Both of these 
appear in the Proceedings. 
