s FUNDAMENTAL CONCEPTIONS 367 



to have been more marked in the work of Kepler than in that of any 

 investigator up to that time. But beyond vague conjectures, Kepler 

 did not try to form any theory to coordinate under a more general 

 concept, the facts, or laws, of planetary motion that he had discov- 

 ered. This task was reserved for Newton. 



It is well known that surmises in relation to a hypothetical attrac- 

 tion emanating from the sun and acting on the planets were more or 

 less vaguely entertained by Kepler, Huyghens, Hooke, Halley, and 

 others in the seventeenth century. They had even conjectured that 

 this attraction might vary inversely as the square of the distance. 

 Hitherto, astronomy had been a formal science, --an attempt merely 

 to define the motions which actually take place. But here we see evi- 

 dence of a desire to refer these motions to some antecedent cause, - 

 a veritable physical origin of them. In fact, the most significant fea- 

 ture of Newton's work is in his discovery that the law of planetary 

 attraction is none other than the terrestrial attraction that acts on 

 bodies at the surface of the earth. 



But as soon as the theory of universal gravitation could be regarded 

 as sufficiently established to warrant extensive labor in its applica- 

 tion, the normal course of astronomical research was resumed. The 

 application of the principle of gravitation to represent the deviations 

 of the orbits of the planets from the exact elliptical form, the nu- 

 merous inequalities in the motion of the moon, the phenomena of 

 planetary satellites, the polar flattening of the earth and planets, was 

 similar in spirit to the attempt to represent the motions of the 

 planets through the geometrical conception of compounded, uniform, 

 circular motion. But it is also true that a greatly increased interest 

 attached to this succession of researches, an interest of broad 

 philosophical scope. This interest is at least twofold. 



In the first place, the representation of the apparent celestial 

 motions through the application of gravitational theorems not only 

 possesses the advantage of coordinating observations in the best 

 possible manner, a thing which previously constituted the whole 

 business of astronomy, but all this work also tends to the firm 

 establishment of a fundamental postulate by means of which not only 

 the motion of one planet, but also all the motions (strictly speaking, 

 the accelerations) of all the planets, satellites, comets, and meteors, 

 could be interpreted and referred to one antecedent cause. 



In the second place, the working out of the consequences of the 

 formula of gravitation emphasized a criterion that is extremely im- 

 portant in estimating weight of evidence in relation to any synthesis 

 concerning natural phenomena. The truth of a theory is measured not 

 simply by success in representing the facts on which it is based. The 

 real test comes in the representation of facts not considered in the 

 original establishment of the theory. Especially satisfactory is it, if 



