262 PRESNEL. 



has enriched the science. Theories are, in general, only 

 methods, more or less happy, of linking together a certain 

 number of facts already known. But when all the new 

 consequences which we can deduce from them are found 

 to agree with experience, they claim a higher importance. 

 This kind of success has not been wanting to Fresnel. 

 His formulas of diffraction include, by implication, a very 

 strange result, which he had not perceived. One of our 

 colleagues* I shall have no need to mention his name, 

 if I say that he has been placed long since among the 

 greatest geometers of this age, as well by a multitude of 

 important labours in pure analysis, as by the most happy 

 applications to the system of the world, and to physics, 

 perceived at a glance the consequence of which I have 

 spoken ; he showed that, in admitting the formulas of 

 Fresnel, the centre of the shadow of an opaque and cir- 

 cular screen ought to be as bright as if the screen did not 

 exist. This consequence, apparently so paradoxical, was 

 subjected to trial by direct experiment, and observation 

 has perfectly confirmed the result of calculation. 



In the long and difficult discussion to which the nature 

 of light has given birth, and of which I have just traced 

 the history, the task of the physicists has been nearly ful- 

 filled ; as to that of the mathematicians, it unhappily still 

 offers some deficiencies to be filled up. I would venture 

 then, if I had the right, to adjure that great geometer 

 to whom optical science owes the important result just 

 mentioned, to try whether the half empirical formulas by 

 which Fresnel has attempted to express the intensities 

 of light reflected under all angles and for all kinds of 

 surfaces, may not be found deducible also from the gen- 

 eral equations of motion of elastic fluids. It remains, 



* Poisson. 



