LIFE AND WORKS OF KEPLER. 103 



Kepler had, in foot, deceived himself by regarding the imijortant 

 advantage obtained over the refractory and stnbboru planet, as one of 

 those decisive victories which forever terminate the contest; the great 

 laws to which he had given ex]>ression, eternally true as they are within 

 due limits, are not rigorous and mathematical. Numerous perturba- 

 tions cause Mars incessantly to deviate from his path, and release him 

 from time to time from the frail bonds in which the exulting calculator 

 thought him entangled forever. For those, it is true, who are enabled 

 to penetrate more deeply, the irregularities in question, once exi)lained 

 ancl foreseen, brilliantly confirm the theory of attraction which they 

 both enlarge and elucidate ; but the premature knowledge of those per- 

 turbations, a necessary consequence of more precise observations, would 

 perhaps, by involving the truth in inextricable embarrassments, have 

 retarded for a long time the progress of celestial mechanics. Kepler 

 would then have been forced, since the elliptical orbit must have been 

 rejected on the same grounds with the circular, to seek by direct means 

 the laws of the disturbed movement, at the risk of exhausting, against 

 insuperable obstacles, all the resources of his penetration and the 

 obstinacy of his i^atience. 



Kepler now conceived the idea of penetrating more deeply into the 

 mysteries of nature and discovering the cause of the movements whose 

 laws he had revealed. After having destroyed forever the old error of 

 obligatorj' circular orbits, he announced the simple and true priucii>le 

 on which rests to-day all ratioiml mechanics : the natural movement 

 of a body is always rectilinear; but unfortunately he adds: ''Pro- 

 vided there be not a soul which directs it," and this restriction mars 

 everything. Neyo uUum motion pcrennam non rectum a I)eo eonditum esse, 

 2)r(vsidio meniali destltntum. There needs, according to this principle, an 

 um-easing force to conduct the planet in its curved orbit, and this force 

 resides in the sun. Kepler affirms this expressly : SoUs igitiir corpus esse 

 fontem virtutis qucc planetas omnes circumcif/it. It is the doctrine ot 

 [N^ewton, or to speak more generally, it is truth. 



Admirers of Kepler have seen in the two phrases just quoted one of 

 his highest titles to renown. On this point I cannot agree with them. 

 lm[)atient of the mystery of the planetary movements, Kepler has here 

 not been faithful to the inspirations of his genius: uncertain and irres- 

 olute, he has attempted on the contrary ail kinds of explanations with- 

 out ado[)ting and vindicating any one of them, and when the tiue idea 

 crossed his mind, he was not able to appreciate or em])loy it. 



After having said that the cause of the movement is in the body of 

 the sun, he supposes that the rotation of that orb is transmitted to the 

 planets and impels them ;, he introduces, further, a magnetic force de- 

 pending on the direction of the axis of the body thus impelled. Views 

 of an extremely vague kind on the nature of attraction lead him more- 

 over to believe that it is inversely proportional to the distance, and it 

 Ijas been remarked that with a \'ery slight modification, his reasoning 

 AYould have conducted him to the true law. Tiiat does not hinder him 

 from believing tliat tlie planet, being sometimes nearer to the sun, some- 

 times more distant from it, must be alternately attracted and repelled. 

 AVith a contradiction which shows beyond all else the uncertainty of his 

 ideas, he asks whether the planet, comprising its force within itself, is 

 not endowed with an active jn^inciple which guides as well as moves it, 

 and without going so far as to accord to it the faculty of reasoning, he 

 bestows upon it a soul., which, instructed as to the route it must tbllow 

 for preserving the <'ternal onler of the universe, directs and nuiintains 

 it therein with imtiaggiug power and exhaustless energy. But how, 



