Jury 27, 1899] 
whole human race, shows us plainly that the scientific 
spirit is at present the mainspring of the life of nations, 
and that it isin the onward march of pure science that 
we are to look for the secret of the growing power of the 
modern world. Whence a series of questions which 
demand more and more the attentions of all. How did 
this taste towards the study of natural philosophy, so 
dear to the ancient philosophers, abandoned for centuries, 
again revive and grow? What are the phases of its 
advance? How appeared the new notions which have 
so deeply modified our ideas on the mechanism of 
nature’s forces? What paths, rich in discoveries, lead 
us gradually unawares to those admirable generalisations 
in accordance with the vast plan foreseen by the founders 
of modern physics? These are the questions which as a 
physicist I intend to inquire into before you. The sub- 
ject is rather abstract, I might say severe. But no other 
has seemed more worthy of your attention during the 
féte which the University of Cambridge celebrates to-day 
in honour of the Lucasian Professorship Jubilee of Sir 
Geo. Gabriel Stokes, who in his fine career has laid a 
master-hand on the very problems which seemed to me 
the most conducive to the progress of natural philosophy. 
The subject is all the more fitted here, as in citing the 
names of those great minds to whom modern science is 
most indebted, we found amongst those who most 
honoured the University of Cambridge—its Professors 
and Fellows—Sir Isaac Newton, Thomas Young, George 
Green, Sir George Airy, Lord Kelvin, Clerk Maxwell, 
Lord Rayleigh, and the memory of that glory which links 
to-day back through the centuries would add lustre to 
the present ceremony. 
Let us then, in a rapid glance of the scientific revival, 
point out the secret but mighty influence which has been 
the directing force of modern physics. I am inclined to 
attribute to the study of light, and to the attraction it has 
for the highest minds, one of the most effective causes of 
the return of ideas towards natural philosophy, and con- 
sider optics as having exercised on the advance of science 
an influence it would be difficult to exaggerate. This in- 
fluence, already clear at the dawn of the experimental 
philosophy under Galileo, grew so rapidly that to-day it 
is easy to foresee a vast synthesis of natural forces 
founded on the principles of the wave theory of light. 
This influence is easy to understand if we reflect that 
light is the way by which knowledge of the exterior 
world reaches our intelligence. It is, in fact, to sight 
that we owe the quickest and most perfect notions of the 
objects around us: our other senses, hearing, feeling, 
also bring their share of learning, but sight alone affords 
us abundant means of simultaneous information such as 
no other sense can. It is, therefore, not surprising that 
light. this lasting link between us and the outward world, 
should intervene with the varied sources of its inner 
constitution to render more precise the observation of 
natural phenomena. Thus each discovery concerning 
new properties of light has had an immediate effect on 
the other branches of human knowledge, and has indeed 
determined the birth of new sciences by affording new 
means of investigation of unexpected power and delicacy. 
Optics are really a modern science. The ancient 
philosophers had no idea of the complexity of what is 
vulgarly called light ; they confounded in the same name 
what is proper to man, and what is exterior. They had, 
however, perceived one of the characteristic properties of 
the link, which exists between the source of light and the 
eye, which receives the impression, “Light moves in a 
straight line.” Common experience had revealed this 
axiom through the observation of the shining trains that 
the sun throws across the skies, piercing misty clouds, or 
penetrating into some dark space. Hence arises two 
empirical notions—the definition of the ray of light, and 
that of the straight line. The one became the basis of 
optics, the other that of geometry. 
NO. 1552, VOL. 60] 
NATURE 
293 
Very little remains to us of the ancient books upon 
optics. Yet we are aware that they knew the reflection 
of the luminous rays on polished surfaces, and the geo- 
metrical explanation of the images formed by mirrors. 
We must wait many centuries until the scientific re- 
vival for a new progress in optics (but then a very con- 
siderable one) opens the new era; it is the invention of 
the telescope. 
The new era begins with Galileo, Boyle and Des- 
cartes, the founders of experimental philosophy. All 
devote their life to meditations on light, colours, and 
forces. Galileo lays the base of mechanics and with the 
refracting telescope that of astro-physics. Boyle im- 
proves experimentation. As to Descartes, he embraces 
with his penetrating mind the whole of natural philosophy; 
he throws away the occult causes admitted by the 
scholastics, and proclaims as a principle that all phe- 
nomena are governed by the laws of mechanics. In his 
system of the universe, light plays a prominent part! : it 
is produced by the waves excited in the subtle matter 
which, according to his view, pervades space. This 
subtle matter (which represents what we call to-day the 
ether) is considered by him as formed of particles in 
immediate contact ; it constitutes thus at the same time 
the vehicle of the forces existing between the material 
bodies which are plunged in it. We recognise the 
famous “vortices of Descartes,” sometimes admired, 
sometimes baffled during the last centuries, but to which 
skilful contemporaneous physicists have rendered the 
importance they deserve. 
Whatever may be the opinions granted to the exact- 
ness of the deductions of this great philosopher, we 
must be struck by the boldness with which he proclaims 
the connection of the great cosmical problems, and for- 
tells the solutions which actual generations did not yet 
entirely accept but drew insensibly to. 
In Descartes’ view the mechanism of light and that of 
gravitation are inseparable ; the seat of corresponding 
phenomena is this subtle matter which pervades the 
universe, and their propagation is performed by waves 
around the acting centres. 
This conception of the nature of light shocked the 
opinions in vogue; it raised strong opposition. Since 
the oldest times it was the habit to imagine the luminous 
ray as the trajectory of rapid projectiles thrown by the 
radiant source. Their shock on the nerves of the eye 
produce vision ; their resistance or changes of speed, 
reflection or refraction. The Cartesian theory had, how- 
ever, some seductive aspects which brought defenders. 
The waves excited on the surface of still water offer so 
clear an image of a propagated motion around a dis- 
turbing centre! On the other hand, do not the sonorous 
impressions reach our ear by waves? Our mind feels 
yet a real satisfaction in thinking that our most sharp 
and delicate organs are both impressed by a mechanism 
of the same nature. 
Yet a serious difference arose. Sound does not 
necessarily travel in straight lines as light does. It 
travels round any object opposed to it, and will follow the 
most circuitous routes with scarcely any loss of strength. 
Physicists were thus divided into two camps. In one 
the partisans of emission, in the other those of the wave 
theory, each system boasting itself superior, and indeed 
each being so in certain respects. Other phenomena 
had to be examined in order to decide between them. 
The chance of discovery brought to view several 
phenomena which ought to have decided in favour of 
wave theory, as was proved a century later ; but the 
simplest truth does not prevail without long endeavour. 
A strange compromise was effected between the two 
systems, helped on by a name great among the greatest, 
and for a century the theory of emission triumphed. 
The tale is a strange one. In 1661 a young scholar, 
1 Le Monde de M. Descartes, ou le Traité de la Lumiére (Paris, 1664). 
