MAN AS THE INTERPRETER OF NATURE. 687 



either by its conformity to known or ascertainable facts as when 

 Kepler determined the elliptic orbit of Mars ; or by the fiulfilment of 

 the predictions it has sanctioned as in the occurrence of an eclipse or 

 an occultation at the precise moment specified many years previously ; 

 or, still more emphatically, by the actual discovery of phenomena till 

 then unrecognized as when the perturbations of the planets, shown 

 by Newton to be the necessary results of their mutual attraction, 

 were proved by observation to have a real existence ; or, as when the 

 unknown disturber of Uranus was found in the place assigned to him 

 by the computations of Adams and Le Verrier. 



"We are accustomed, and I think most rightly, to speak of these 

 achievements as triumphs of the human intellect. But the very phrase 

 implies that the work is done by mental agency, and the coincidence 

 of its results with the facts of observation is far from proving the in- 

 tellectual process to have been correct. For we learn, from the con- 

 fessions of Kepler, that he was led to the discovery of the elliptic or- 

 bit of Mars by a series of happy accidents, which turned his erroneous 

 guesses into the right direction ; and to that of the passage of the 

 radius vector over equal areas in equal times by the motion of a whirl- 

 ing force emanating from the sun, which we now regard as an entirely 

 wrong conception of the cause of orbital revolution. It should always 

 be remembered, moreover, that the Ptolemaic system of astronomy, 

 with all its cumbrous ideal mechanism, did intellectually represent all 

 that the astronomer, prior to the invention of the telescope, could see 

 from his stand-point the earth* with an accuracy which was proved by 

 the fulfilment of his anticipations. And in that last and most memo- 

 rable prediction, which has given an imperishable fame to our two 

 illustrious contemporaries, the inadequacy of the basis afforded by 

 actual observation of the perturbations of Uranus required that it 

 should be supplemented by an assumption of the probable distance of 

 the disturbing planet beyond, which has been shown by subsequent 

 observation to have been only an approximation to the truth. 



Even in this most exact of sciences, therefore, we cannot proceed a 

 step without translating the actual phenomena of Nature into intel- 

 lectual representations of those phenomena, and it is because the New- 

 tonian conception is not only the most simple, but is also, up to the 

 extent of our present knowledge, universal in its conformity to the 

 facts of observation, that we accept it as the only scheme of the uni- 

 verse yet promulgated which satisfies our intellectual requirements. 



When, under the reign of the Ptolemaic system, any new inequal- 

 ity was discovered in the motion of a planet, a new wheel had to be 

 added to the ideal mechanism as Ptolemy said, "to save appear- 

 ances." If it should prove, a century hence, that the motion of Nep- 

 tune himself is disturbed by some other attraction than that exerted 

 by the interior planets, we should confidently expect that not an ideal 

 but a real cause for that disturbance will be found in the existence of 



