KEPLER 191 



themselves. It may be true, he says, that during an eclipse 

 there is some change, some alteration in this force, and hence 

 in the direction of the planetary motion. This may, he con- 

 jectures, explain why the orbit of the planets is not a perfect 

 circle. He just misses the splendid intuition of Newton that 

 this attractive force being reciprocal, when one body circles 

 about another it pulls this other out from the centre, so that 

 its own path becomes an ellipse. 



His fancy knows no rein : thus will he endeavour to ex- 

 plain why it is that the earth and other planets do not fall into 

 the sun. Some force must hold them in their course. Kepler 

 is a poet ; he is dowered with one of the most vivid of imagina- 

 tions, quite as wonderful, we may believe, as that of any Milton 

 or even Shakespeare. For him the universe is alive ; the earth 

 is a great animal the tides are its breath. He considers the 

 spheres, the stars, as inhabitants of the ether. In the ether they 

 are born, in the ether they live like plants and animals ; they 

 are the butterflies of space. They maintain their motion by 

 virtue of an animal force which resides within them. 



His exuberant imagination leads him astray. He has per- 

 ceived clearly enough the first part of the first law of motion, 

 that bodies at rest tend to remain at rest so long as they are not 

 acted upon by any exterior force. He does not perceive that 

 the planets need no urging force, that once set going and un- 

 impeded, they will go on for ever. It is not given to any one 

 human mind to grasp the truth whole. Newton could not. 

 What wonder if Kepler failed ? The minds even of the greatest 

 of each generation are bound up, limited, in some sense per- 

 verted, by the preconceptions born with them. When the 

 tireless, teeming brain of Kepler was searching, searching, 

 searching for causes, reasons, effects, no science of mechanics 

 existed. It is this alone, we may conceive, which withheld 

 from him the glory of having demonstrated what he had divined, 

 from having reduced all planetary motion to a single principle, 

 from having revealed the whole solar system as the workings 

 of a vast machine. 



No matter, it is enough ; the paths of the planets are now 

 absolutely known, proved. The Coppernican idea has received 

 a perfectionment that amounts almost to a demonstration. 

 Hardly any longer will it be possible for any timid Peter to 

 deny the truth. One step more and it will no longer be possible. 



