XX11 INTRODUCTION. 



answering to a given orbit is much easier than 

 the converse of finding the orbit from knowing 

 the force ; accordingly the subject of the inverse 

 problem may be passed over.* So may the great 

 head of disturbing forces f , but the interesting his 

 tory of the problem of three bodies may without 

 difficulty be comprehended. J The investigation, 

 however, of Lagrange and Laplace of the prin 

 ciple upon which the stability of the system 

 depends, must be taken upon the results without 

 entering into the steps of the process. But 

 the wonderful anticipation by Newton of sub 

 sequent discoveries may be generally understood 

 and appreciated. || The subject of the attraction 

 of masses, spherical and others, may be regarded 

 as not coming within this elementary view of the 

 work.^[ The application of dynamical principles 

 to the rays of light **, and the general statement 

 respecting the Newtonian discoveries, and the con 

 stitution of the universe, may easily be followed.f f 

 Nor can there be great difficulty in understanding 

 the explanation given under the Third Book, of 

 the effects of attractive forces upon the figures of 

 the heavenly bodies, the motions of comets, so far 



* P. 76, to 87. t P - 87- to 108. 



.t P. 93, 94. 108. to 112. P. 112. to 124. 



j| P. 132. to 134. fP. 134. to 150. 



** P. 150. to 153. ft P- 153. to 160. 



