486 PHOTOGRAPHY IN THE SEEVICE OF ASTRONOMY. 



ought to permit us to hope that after thirty or forty years of observa- 

 tions of the uew phiuet, it may be possible to employ them in their 

 tnrn in the discovery of that which follows it in order of distance from 

 tbe sun."* 



He continues thus : " We will unhappily soon fall upon stars invisi- 

 ble, on account of their immense distance from the sun, but whose orbits 

 will be completed, in the course of centuries, by being traced with great 

 exactness, by means of the theory of secular inequalities.'- More than 

 forty years have elapsed since the discovery of Neptune without real- 

 izing the hope of Le Verrier. Tbe fact is that his formulse represent 

 always with precision not only the observations of Neptune made since 

 1840, but also some observations much more ancient (Lalande had come 

 upon the planet twice, in 1795, and had entered it in his catalogue as a 

 star of the eighth magnitude). We do not know then on what to rest, 

 to renew the prodigious discovery of Le Yerrier, which already itself 

 had been possible only on account of a ha^jpy concourse of circumstances. 

 It is this which we can not prevent ourselves from remembering in read- 

 ing the masterly exposition which Mr. Tisserand has made in the his- 

 tory of the discovery of Neptune in the first tome of his " Treatise on 

 Celestial Mechanics" which appeared a few months since. 



The trans-Neptunian planet, if it exists, will be found perhaps at a 

 distance very great, surpassing more than one hundred times the radius 

 of the terrestrial orbit, or else its mass is relatively small, and the action 

 which it exercises will not make accusation against it till after a long 

 period. Let us not forget that Neptune has scarcely traversed one quar- 

 ter of his orbit since the epoch of discovery. It may be possible even 

 that the action of a mass relatively large may remain for a long time 

 hidden from us, in being confounded with that of the other planets. 



There are then few chances for discovering the hypothetical star by 

 virtue alone of the law of Newton. It is necessary rather to count on 

 the happy chance of recognizing it among stars of the twelfth or thir- 

 teenth magnitude, among which it may be lost. All these things did 

 not prevent Mr. David P. Todd from constituting himself the prophet 

 of the trans-Neptunian planet, for which he entered upon a search 

 since 1874 by the systematic exploration of certain regions of the sky.t 

 During the winter of 1877-'78 he employed in this exploration the great 

 refractor of the Washington observatory. He closed in placing his hope 

 in photography, which appears called to render this class of researches 

 much more easy. Mr. Todd founds his conviction of the existence of 

 the planet upon the examination of the last residuals of the Tables of 

 Uranus, to which he has applied a very simple graphical process indi- 



* Galle having proposed for the new planet the name of Janus, Le Verrier replied 

 to him : " The name of Janus would indicate that this planet is the last of the solar 

 system, which there is no reason to believe." 



t "Accouut of a specnlative and practical search for a trans-Neptunian planet," 

 1880. {Proceedings of the American Academy of Sciences, 1880-'86.) 



