262 _ FRESNEL. | 
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. 
