198 FRESNEL. 



complained of complexity. But again, some time after, 

 in the hands of Newton, these motions, complex in ap- 

 pearance, became the basis of the greatest discoveries of 

 modern times, of a principle as simple as it is fertile ; 

 they served to prove that every planet is governed in its 

 elliptic course by a simple force, by an attraction emanat- 

 ing from the sun. 



Those observers again, who, in their turn refining 

 upon Kepler, showed that simple elliptic motions would 

 not suffice to represent the true paths of the planets, 

 did not simplify the science. But besides that the 

 derangement (known under the name of perturbations) 

 would not the less have existed if, in the dislike of all 

 complexity, we had obstinately determined to shut our 

 eyes to them, I ought to "ay, that in studying them with 

 cai'e we have been conducted, among many other impor- 

 tant results, to the means of comparing the masses of the 

 different bodies of which our solar system is composed ; 

 and that if at the present day we know, for example, that 

 it requires not less than 350,000 times the globe of the 

 earth to form a weight equal to that of the sun, we owe 

 it to the observation of those very small inequalities, 

 which those would certainly have neglected, who at all 

 risks would admit nothing but simple phenomena. 



Without extending these remarks farther, I may then 

 admit that optics would be a more easy science, more at 

 the command of the generality of men, more susceptible 

 of demonstration in public lectures, before the extension 

 of it which has been made in our times. But this exten- 

 sion is a real source of riches ; it has given occasion for 

 the most curious applications ; it has thence afforded 

 those indications of impossibilities in certain theories of 

 light, which may claim to rank among discoTeries ; for 



