228 PROCEEDINGS OF THE AMERICAN ACADEMY 



X. 



TELESCOPIC SEARCH FOR THE TRANS-NEPTUNIAN 



PLANET. 



By David P. Todd, M. A. 



Communicated June 10, 1885. 



In the twentieth volume of the American Journal of Science, at page 

 225, I gave a preliminary account of my search, theoretic and prac- 

 tical, for the trans-Neptunian planet. I say the trans-Neptunian planet, 

 because I regard the evidence of its existence as well founded, and 

 further, because, since the time when I was engaged upon this search, 

 nothing has in the least weakened my entire conviction as to its exist- 

 ence in about that part of the sky assigned ; while, as is well known, 

 the independent researches in cometary perturbations by Professor 

 •Forbes conducted him to a result identical with my own, — a co- 

 incidence not to be lightly set aside as pure accident. 



That five years have elapsed since this coincidence was remarked, 

 and the planet is still unfound, is not sufficient assurance to me that 

 its existence is merely fanciful. In so far as I am informed, this spot 

 of the sky has received very little scrutiny with telescopes competent 

 to such a search ; and most observers finding nothing would, I sus- 

 pect, prefer not to announce their ineffective search. 



The time has now come when this search can be profitably under- 

 taken by any observer having the rare combination of time, enthusi- 

 asm, and the necessary appliances. Strongly marked developments in 

 astronomical photography have been effected since this optical search 

 was conducted ; and the capacity of the modern dry -plate for the regis- 

 try of the light of very faint stars makes the application of this method 

 the shortest and surest way of detecting any such object. Nor is this 

 purely an opinion of my own. But the required apparatus would be 

 costly ; and the instrument, together with the services of an astronomer 

 and a photographer, would, for the time being, be necessarily devoted 

 exclusively to the work. While, however, the photographic search 

 mif^ht have to be ended, with a negative result, in so far as the trans- 

 Neptunian planet is concerned, there would still remain the series of 



