296 
NATURE 
[Jury 27, 189y 
of light. Looked at from this point of view, the history 
of optics acquires a considerable philosophical import- 
ance ; it becomes the history of the successive progress 
of our knowledge on the means that nature employs to 
transmit movement and force to a distance. 
The first idea which came to the mind of man (in the 
savage state) to exercise his force beyond his reach is 
the throwing of a stone, of an arrow or of some projec- 
tile ; this is the germ of the theory of emission. This 
theory corresponds to a philosophical system which 
assumes an empty space in which the projectile moves 
freely. At a more advanced degree of culture, man 
having becomea physicist, has had the more delicate idea 
of the transmission of movement by waves, suggested at 
first by the study of waves, afterwards by that of sound. 
This second way supposes, on the other hand, that 
space is a plenum ; there is no longer here transport of 
matter ; particles oscillate in the direction of propagation, 
and it is by compression or rarefaction of a continuous 
elastic medium that movement and force are trans- 
mitted. Such has been the origin of the theory of 
luminous waves ; under this form it could only represent 
a part of the phenomena ; it was therefore insufficient. 
But geometers and physicists before Fresnel did not 
know of any other undulatory mechanism in a continuous 
medium. 
The great discovery of Fresnel has been to reveal a 
third mode of transmission quite as natural as the pre- 
ceding one, but which offers an incomparable richness of 
resources. These are the waves of transverse vibrations 
excited in an incompressible continuous medium, those 
which explain all the properties of light. 
In this undulatory mode the displacement of particles 
brings into play an elasticity of a special kind; this is 
the relative slipping of strata concentric to the disturb- 
ance which transmits the movement and the effort. The 
character of these waves is to impose on the medium no 
variation of density as in the system of Descartes. The 
richness of resource mentioned above depends upon the 
fact that the form of the transverse vibration remains 
indeterminate, and thus confers on waves an infinite 
variety of different properties. 
The rectilinear, circular and elliptical forms characterise 
precisely the polarisations, so unexpected, which Fresnel 
discovered, and by the aid of which he has so admirably 
explained the beautiful phenomena of Arago produced by 
crystallised plates. 
The possible existence of waves which are propagated 
without change of density, has profoundly modified the 
mathematical theory of elasticity. Geometers found 
again in their equations, waves having transverse vibra- 
tions which were unknown to them ; they learnt besides, 
from Fresnel, the most general constitution of elastic 
media, of which they had not dreamt. 
It is in his admirable memoir on double refraction that 
this great physicist set forth the idea that in crystals the 
elasticity of the ether, ought to vary with the direction, 
an unexpected condition and one of extreme importance, 
which has transformed the fundamental bases of mole- 
cular mechanics ; the works of Cauchy and Green are 
the striking proofs of it. From this principle Fresnel 
concluded the most general form of the surface of the 
luminous wave in crystals, and found (as a particular 
case) the sphere and ellipsoid that Huygens had assigned 
to the Iceland spar crystal. This new discovery excited 
universal admiration among physicists and geometers ; 
when Arago came to expound it before the Académie 
des Sciences, Laplace, who had been such a long time 
hostile, declared himself convinced. Two years later 
Fresnel, unanimously elected a member of the Academy, 
was elected with the same unanimity foreign member of 
the Royal Society of London ; Young himself transmitted 
to him the announcement of this distinction, with personal 
testimony of his sincere admiration. 
NO. 1552, VOL. 60] 
The definite foundation of the undulatory theory im- 
poses the necessity of admitting the existence of an 
elastic medium to transmit the luminous movement. But 
does not all transmission to a distance of movement or 
of force imply the same condition? To Faraday is due 
the honour of having, like a true disciple of Descartes 
and Leibnitz, proclaimed this principle, and of having 
resolutely attributed to reactions of surrounding media 
the apparent action at a distance of electrical and mag- 
netic systems. Faraday was recompensed for his bold- 
ness by the discovery of induction. 
And since induction acts even across a space void of 
ponderable matter, one is forced to admit that the active 
medium is precisely that which transmits the luminous 
waves, the ether. 
The transmission of a movement by an elastic medium 
cannot be instantaneous ; if it is truly luminous ether 
that is the transmitting medium, ought not the induction 
to be propagated with the velocity of luminous waves ? 
The verification was difficult. Von Helmholtz, who 
tried the direct measurement of this velocity, found, as 
Galileo formerly, for the velocity of light a value 
practically infinite. 
But the attention of physicists was attracted by a 
singular numerical coincidence. The relation between 
the unity of electrostatic quantity to the electro-magnetic 
unit is represented by a number precisely equal to the 
velocity of light. 
The illustrious Clerk Maxwell, following the ideas of 
Faraday, did not hesitate to see in the relationship the 
indirect measure of the velocity of induction, and by a 
series of remarkable deductions he built up this cele- 
brated electro-magnetic theory of light, which identifies 
in one mechanism three groups of phenomena completely 
distinct in appearance, light, electricity, and magnetism. 
But the abstract theories of natural phenomena are 
nothing without the control of experiment. 
The theory of Maxwell was submitted to proof, and 
the success surpassed all expectation. The results are too 
recent and too well known, especially here, for it to be 
necessary to insist upon them. 
A young German physicist, Henry Herz, prematurely 
lost to science, starting from the beautiful analysis of 
oscillatory discharges of Von Helmholtz and Lord Kelvin, 
so perfectly produced electric and electro-magnetic waves, 
that these waves possess all the properties of luminous 
waves ; the only distinguishing peculiarity is that their 
vibrations are less rapid than those of light. 
It follows that one can reproduce with electric dis- 
charges the most delicate experiments of modern optics 
—reflection, refraction, diffraction, rectilinear, circular. 
elliptic polarisation, &c.. But I must stop, gentlemen. 
I feel that I have assumed too weighty a task in en- 
deavouring to enumerate the whole wealth which waves 
of transverse vibrations have to-day placed in our hands. 
I said at the beginning that optics appeared to me 
to be the directing science in modern physics. 
If any doubt can have arisen in your minds, I trust 
this impression has been effaced to give place to a 
sentiment of surprise and admiration in seeing all that 
the study of light has brought of new ideas on the 
mechanism of the forces of nature. 
It has insensibly restored the Cartesian conception of 
a single medium refilling space, the seat of electrical, 
magnetic and luminous phenomena ; it allows us to fore- 
see that this medium is the depositary of the energy 
spread throughout the material world, the necessary 
vehicle of every force, the origin even of universal 
gravitation. : 
Such is the work accomplished by optics : 
the greatest thing of the century ! 
The study of the properties of waves, viewed in every 
aspect, 1s therefore, at the present moment, the most 
fertile study. 
it is perhaps 
