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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 say, that in studying them with 
care we have been conducted, among many other inipor- 
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 s¢mple 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 discoveries ; for 
