On the 16th 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 themselves 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.