JuLy 27, 1899] 
NATURE 
295 
impartial discussion of opposite systems ; it appears as 
the painting of the suffering of a mighty genius, worried 
by doubt, now led away by the seductive suggestions of 
his imagination, now recalled by the imperious require- 
ments of logic. It is a drama: the everlasting struggle 
between love and duty ; and duty won. 
Such, I take it, is the inner genesis of the theory of 
fits—a strange mingling of two opposite systems, It 
was much admired, presented, as it was, by the great 
mathematician, who had the glory of submitting the 
motions of all celestial bodies to the one law of universal 
gravitation. 
To-day this theory is abandoned ; it is condemned by 
the experimentum crucis of Arago, realised by Fizeau and 
Foucault. One ought, however, to acknowledge that it 
has constituted a real progress by the precise and new 
notions which it contains. ‘The ray of light, considered 
up till then, was simply the trajectory of a particle in 
rectilinear motion ; the ray of light, such as Newton de- 
scribed it, possesses a regular periodic structure, and the 
period or interval of fits, characterises the colour of the 
vay. This is an important result. It only requires a more 
suitable interpretation to transform the luminous ray into 
a vibratory wave; but we had to wait a century, and Dr. 
Thomas Young, in 1801, had the honour of discovering it. 
Resuming the study of thin plates, Thomas Young 
shows that everything is explained with extreme sim- 
plicity, if it be supposed that the homogeneous luminous 
ray is analogous to the sonorous wave produced by a 
musical sound; that the vibrations of ether ought to 
compose—that is to say, to interfere—according to 
the expression that he proposes as to their mutual 
actions. 
Although Young had taken the clever precaution of 
supporting his views by the authority of Newton,! the 
hypothesis found no favour ; his principle of interference 
led to this singular result that light added to light could, 
in certain cases, produce darkness, ag paradoxical result 
contradicted by daily experience. The only verification 
that Young brought forward was the existence of dark 
rings in Newton’s experiment, darkness due, according 
to him, to the interference of waves reflected on the two 
faces of the plate. But as the Newtonian theory inter- 
preted the fact in a different manner, the proof remained 
doubtful ; an experimentum crucis was wanting. Young 
did not have the good success to obtain it. 
The theory of waves relapsed then once more into the 
obscurity of controversy, and the terrible argument of 
the rectilinear propagation was raised afresh against it. 
The most skilled geometers of the period—Laplace, Biot, 
Poisson—naturally leaned to the Newtonian opinion ; 
Laplace in particular, the celebrated author of the 
““ Mecanique Celeste,” had even taken the offensive. He 
was going to attack the theory of waves in its most 
strongly fortified entrenchments, which had been raised 
by the illustrious Huygens. 
Huygens, indeed, in his “‘ Traité de la Lumiére,” had 
resolved a problem before which the theory of emission 
had remained mute ; that is to say, the explanation of 
the double refraction of Iceland spar: the wave theory 
(on the contrary) reduced to the simplest geometrical 
construction the path of the two rays, ordinary and extra- 
ordinary ; experiment confirmed the results in every 
point. Laplace succeeded in his turn (with the help of 
hypotheses of the constitution of luminous particles) to ex- 
plain the path of these strange rays. The victory of the 
theory of particles then appeared complete; a new 
phenomenon arrived also appropriately to render it 
striking. 
Malus discovered that a common ray of light reflected 
under a certain angle acquired unsymmetrical properties 
similar to those rays from a crystal of Iceland spar. He 
1 The Bakerian Lecture, ‘‘On the Theory of Light and Colours.” By 
Thomas Young. PAzl. Trans. of the R.S. for the year 1802. 
NO. 1552, VOL. 60] 
explained this phenomenon by an orientation of the 
luminous molecule, and, consequently, named this light 
polarised light. This was a new success for emission. 
The triumph was not of long duration. In 1816 a 
young engineer, scarcely out of the Ecole Polytechnique, 
Augustin Fresnel, confided to Arago his doubts on the 
theory then in favour, and pointed out to him the experi- 
ments which tended to overthrow it. 
Supporting himself on the ideas of Huygens, he 
attacked the formidable question of rays and shadows, 
and had resolved it: all the phenomena of diffraction 
were reduced to an analytical problem, and observations 
verified calculation marvellously. He had, without know- 
ing it, rediscovered Young’s reasonings as well as the 
principle of interference ; but more fortunate than he, 
he brought the experdmentum crucis—the two-mirror 
experiment; there, two rays, issuing from the same 
source, free from any disturbance, produced when they 
met, sometimes light, sometimes darkness. The illus- 
trious Young was the first to applaud the success of his 
young rival, and showed him a kindness which never 
changed. 
Thus, thanks to the use of two-mirror experiment, 
the theory of Dr. Young (that is to say, the complete 
analogy of the luminous ray and the sound wave) is firmly 
established. 
Moreover, Fresnel’s theory of diffraction shows the 
cause of their dissimilarity; light is propagated in 
straight lines because the luminous waves are extremely 
small. On the contrary, sound is diffused because the 
lengths of the sonorous waves are relatively very great. 
Thus vanished the terrible objection which had so 
much tormented the mind of great Newton. 
But there remained still to explain another essential 
difference between the luminous wave and the sonorous 
wave ; the latter undergoes no polarisation. Why is the 
luminous wave polarised ? 
The answer to this question appeared so difficult that 
Young declared he would renounce seeking it. Fresnel 
worked more than five years to discover it ; the answer 
is as simple as unexpected. The sound wave cannot be 
polarised because the vibrations are longitudinal ; light, 
on the other hand, can be polarised because the vibra- 
tions are transverse, that is to say, perpendicular to the 
luminous ray. 
Henceforth the nature of light is completely estab- 
lished, all the phenomena presented as objections 
to the undulatory theory are explained with marvellous 
facility, even down to the smallest details. 
I would fain have traced by what an admirable suite of 
experiment and reasoning Fresnel arrived at this dis- 
covery, one of the most important of modern science : 
but time presses. 
It has sufficed me to explain how very great the 
difficulties were which he had to overcome in order to 
establish it. 
I hasten to point out its consequences. 
You saw, at starting, the purely physiological reasons 
which make the study of light the necessary centre of 
information gathered by human intelligence. You judge 
now, by the march of this long development of optical 
theories, what preoccupations it has always caused to 
powerful minds interested in natural forces. Indeed, all 
the phenomena which pass before our eyes involve a 
transmission to a distance of force or movement ; let 
the distance be infinitely great, as in celestial space, or 
infinitely small, as in molecular intervals, the mystery is 
the same. But light is the agent which brings us the 
movement of luminous bodies ; to fathom the mechanism 
of this transmission is to fathom that of all others, and 
Descartes had the admirable intuition of this when he 
comprehended all these problems in a single mechanical 
conception: here is the secret bond which has always 
attracted the physicists and geometers towards the study 
