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are founded on the precise estimate of the first places
which he obtained at the college, in theme or in transla-
tion, would never have imagined that Augustine Fresnel
would become one of the most distinguished savants of
our epoch. As to his young comrades, they had, on the
contrary, judged with that sagacity which rarely deceives
them ; they called him " the genius." This pompous title
was unanimously accorded him on account of the experi-
mental researches (I may be allowed this expression, it is
but just) to which he devoted himself at the age of nine
years, whether for determining the relative length and
bore which give the greatest power to the little elder-
wood popguns which children use in their play, or in de-
termining which are the wood-, dry or green, which are
best to use in making bows, under the double considera-
tion of elasticity and strength. The physicist of nine
years old had, indeed, executed this little work with so


much success, that the toys, hitherto very inoffensive, had
become dangerous arms, which he had the honour of" see-
ing proscribed by an express resolution of the assembled
parents of all the combatants.

In 1801, Fresnel, aged thirteen, quitted the paternal
hearth, and went to Caen with his elder brother. The
central school of this town, where the instruction has
always been creditable, presented then a reunion of pro-
fessoi's of the rarest merit. The excellent lessons ia
mathematics from M. Quenot, the course of general gram-
mar and logic from the Abbe de la Riviere, eminently
contributed to develop in the young pupil that sagacity,
that rectitude of mind, which guided him afterwards so
happily in the apparently inextricable labyrinth of natural
phenomena which he succeeded in clearing. The com-
munication of knowledge is, of all the benefits which we
receive in our youth, that of which a generous heart pre-
serves the deepest remembrance. Hence the gratitude
which Fresnel had felt towards his worthy professors at
Caen was always lively and respectful. The central
schools themselves always occupied a large share of his
recollections ; and I have some reason to believe that
many reminiscences of these ancient institutions would
have been found in a plan of study which he wished to

Fresnel entered the Polytechnic School at the age of
sixteen and a half, where his eldest brother had preceded
him one year before. His health was at that time ex-
tremely weak, and gave reason to fear that he would be
unable to support the fatigue of so I'ough a noviciate ;
but that feeble body enclosed the most vigorous soul, and
in all things the firm will to succeed is already half the
success ; moreover, the dextex-ity of Fresnel in the


graphic arts was nearly unequalled, and on this ground
he could fully compete with the cleverest of his com-
rades, even whilst imposing upon him far less work in a
day. Wiien Fresnel went through the course at the
Polytechnic School, a savant, Avhose zeal age has not
cooled, — whom the Academy of Sciences has the happi-
ness to number amongst its most active and most assidu-
ous members, and whom, as he is listening to me, I will
only designate by the simple title of the chief of living
geometers, — fulfilled the duties of examiner. In the
course of the year 1804, he proposed to the pupils, as a
subject of competition, a geometrical question. Several
solved it ; but the solution of Fresnel particularly struck
the attention of our colleague ; for superior men enjoy
the happy privilege of discovering, even from slight
indications, the talents which will shine brightly. M.
Legendre (his name escapes from my lips) complimented
the young prize-man publicly. Proofs of encourage-
ment coming from so high a quarter revealed to Fresnel,
perhaps for the first time, the secret of his own merit,
and conquered an excessive feeling of mistrust, which
with him produced the most vexatious results, because it
prevented him from attempting new paths.

On leaving the Polytechnic School, Fresnel passed
into the department of the " ponts et chaussees." When
he had obtained the rank of ingenieur ordinaire, he was
sent into the department of the Vendee, where the gov-
ernment, desirous to efface the traces of our deplorable
civil discord, raised up all that war had thrown down,
opened communications destined to give life to the
country, and laid the foundations of a new town. Every
pupil, whatever may be the career he is about to enter
upon, awaits with the most eager impatience the instant



at which he may give up that title. To him, in four-
and-twenty hours, the appearance of the world becomes
completely changed ; he has hitherto received instruc-
tion ; he is going to create it. His future seems, more-
over, to promise him all that a century may have offered
in the way of brilliant occurrences to some few individ-
uals favoured by fate.

Few engineers, for example, receive their diplomas
without believing themselves from this moment called
(like new " Riquets ") either to join the ocean to the
Mediterranean by a great canal which will carry mer-
chantmen even to the centre of a kingdom, or to trace
on the slope of the Alps the winding and bold road
whose summit is lost amidst eternal snow, but which the
traveller nevertheless will face even in the depth of
winter. One has conceived the hope of ornamenting
the capital with one of those light, and at the same time
steady bridges, where the bold chisel of a David may
some day come to animate the marble ; another, remod-
elling the gigantic works of Cherbourg, arrests tempests
at the entrance of roadsteads, provides useful harbours
for merchantmen, associates himself finally with the
glory of the national squadrons by furnishing them with
ncAv means of attack and defence. The less ambitious
have dreamt of improving the course of the principal
rivers, and rendering their waters deeper and less rapid
by means of embanknaents ; of checking those moving
mountains which, under the name of sandhills, gradually
invade rich countries and transform them into sterile

I will not venture to affirm that, notwithstanding the
extreme moderation of his desires, Fresnel entirely
escaped these happy dreams of youth. At all events the


sequel was unexpected. To level small portions of
road ; to seek, in the countries placed under his superin-
tendence, for beds of flint; to preside over the extraction
of the materials ; to see to their deposition on the road,
or in the wheel ruts ; to execute, here and there, a
bridge over the irrigation drains ; to reestablish some
metres of bank which the torrent has carried away in its
progress ; to exercise principally an active surveillance
over the contractors ; to verify their accounts ; to esti-
mate scrupulously their woi'ks, — such were the duties,
very useful, though not very lofty, not very scientific,
which Fresnel had to fulfil during from eight to nine
years in Vendee, in Drome, and in Ille et Vilaine.
How heavily must a mind of such power have been
affected, when he compared the use which he might
have made of those hours which pass away so quickly,
with the way in which they were being spent ! But
with Fresnel conscientiousness was always the foremost
part of his character, and he constantly performed his
duties as an engineer with the most rigorous scrupulous-
ness. The mission to defend the revenues of the state,
to obtain for them the best employment possible, ap-
peared to his eyes in the light of a question of honour.
The functionary, whatever might be his rank, who sub-
mitted to him an ambiguous account, became at once the
object of his profound contempt. Fresnel could not
comprehend the conduct to which persons, in other
respects very estimable, believe themselves bound some-
times by an esprit de corps. All fraternity ceased for
him, notwithstanding the similarity of title and uniform,
as soon as any one lost a probity free of suspicion.
Under such circumstances the habitual gentleness of his
manners disappeared, and gave place to a sternness, I


will even say a roughness, which in this age of conces-
sions drew upon him numerous vexations.

The purely speculative opinions of a studious man
concerning the political organization of society, must
generally be of too little interest to the public to render
their mention necessary ; but the influence which they
exercised on the career of Fresnel will not allow me to
be silent upon them.

Fi'esnel, like so many good men, associated himself
deeply in 1814 with the hopes to which the return of
the Bourbon family gave rise. The charter of 1814,
executed without retrospective effect, appeared to him to
contain all the germs of a wise liberty. He saw in it
the aui'ora of a political regeneration which would, with-
out a check, extend itself from France over all Europe.
His patriotic spirit was excited with the idea that our
beautiful country was about to exercise such a pacific
influence over the good of nations. If, during the Im-
perial dynasty, the great events of Austerlitz, of Jena, of
Friedland, had not strongly excited his imagination, it
was solely because they appeared to him destined to per-
petuate that despotism under which France at that time
bent. The disembarkment at Cannes, in 1815, appeared
to him an attack on civilization ; and thus, without being
hindered by the disordered state of his health, he was
anxious to go and join one of the detachments of the
royal army of the south. Fresnel flattered himself with
the hope of meeting only with men of his own disposi-
tion, if I may judge from the painful impression which
he experienced at his first interview with the general
under whose orders he went to place himself. Touched
by the invalid appearance of the new soldier, the general
testified his surprise that in such a condition he should


expose himself to the fatigues and dangers of a civil war.
" Your superiors, Sir," said he, " have enjoined on you
this expedition." "No, general," he replied, "I have
taken no advice but my own." " I pray you tell me
without reserve, has any one threatened you with not
paying your appointments ? " " No such threat has been
made ; my appointments have been regularly paid."
" Very well ; I ought, between ourselves, to warn you
that you can here reckon only on what may be got by
chance." " I have reckoned my own resources ; I
neither hope nor desire any other recompense. I pre-
sent myself to you to fulfil my duty." " I admire you,
Sir ; it is thus that every good servant of the royal cause
ought to think and act ; I participate in your honourable
sentiments ; you may reckon on my good will."

That good will, in fact, did not fail ; and the questions
which at first had been painful to Fresnel, showed solely
that his questioner, less a novice in the ways of the
world, knew by experience that a popular gathering,
under whatever colour it may show itself, includes more
than a few individuals who under high pretensions con-
ceal personal interests.

Fresnel returned to Nyons, his usual residence, almost
dying. The news of the events of the Palud had pre-
ceded him. The populace (we know what this term sig-
nifies in the south) offered him a thousand insults. A
few days afterwards an imperial commissary declared
his deprivation of his office, and placed him under the
surveillance of the police. Far be it from me to ex-
tenuate the odious nature of such a transaction. I ought,
however, to say that it was executed without need-
less rigour, and that Fresnel obtained permission to go to
Paris; that he lived there without being disturbed; that


he was able to renew his acquaintance with his old
fellow students, and to prepare for those scientific re-
searches which he designed to pursue in the retreat
where his younger years had been passed. At this time
Fresnel had but a very confused idea of the brilliant dis-
coveries, which, in the early years of the present cen-
tury, entirely changed the aspect of optical science.

fresnel's first scientific papers.

The first memoir on science which Fresnel drew up,
dates from this same year, 1814. It was an essay whose
object was to rectify the explanation, considered as
imperfect, of the phenomenon of the aberration of the
light of the stars hitherto generally followed in elemen-
tary works. Both geometry and physical science equally
bore out this new administration ; but, unfortunately, it
too closely resembled that already given by Bradley him-
self and by Clairault. I say unfortunately, because, if
we should suppose that such coincidences are pleasing to
the self-love of a dehutant, or stimulate his zeal, it would
be a mistake. On the other hand, an author may sup-
port with philosophy, I admit, the unpleasant fact of
having uselessly employed his powers for years in the
search after a truth already long since established ; he
may give up, with the best grace, the flattering hope of
seeing his name associated with some brilliant discovery ;
but might he not feel much more disquieted when there
was ground to fear, that from mere ignorance of the
existence of prior researches, of which no one dreamed,
he might stand charged with plagiarism ? when he might
apprehend that an irreproachable character was no safe-
guard against such imputations ? The public, notwith-
standing the most express denials, will always believe


that an author knows all that he might be supposed to
know ; and the right with which it is invested, to ti-eat
with implacable severity those vVho knowingly borrow
from the labours of their predecessors, is the origin of
more than one act of injustice. Thus, Lagrange has
recounted that in his youth he experienced just such a
profound mortification, on finding, by accident, in the
works of Leibnitz, an analytical formula which he had
completely forgotten, and of which he had spoken to the
Academy of Turin as a discovery of his own. From
that day he had nearly renounced altogether the study
of mathematics. The demonstration of aberration was
a matter of too little importance to inspire Fresnel with
a similar discouragement ; and besides, he had not print-
ed it ; but this circumstance rendered him extremely
timid ; and subsequently he never published any memoir
without assuring himself by the testimony of some of his
friends, to whom the academical collections were more
familiar, that he had not. according to a popular proverb
which he habitually adopted, "broken through open
doors." *

* It is much to be regretted that this early production of Fresnel
should not have been preserved — more especially when we recollect
that the theoretical explanation of the aberration of light, though ap-
parently well given by Clairault and others, was for a long time by
no means clearly apprehended, and far from being exempt from all
necessity for further elucidation. In proof of this it may suffice to
allude to the fact that, on the occasion of the transit of Venus in
1769, two eminent astronomers, Bliss and Hornsby, calculated the
efFect of aberration as accelerating the phases of the transit, while
Professor Winthrop, of Cambridge, U. S., argued that it ought to be
that o^ retarding them. Other discrepancies of opinion in past times
might also be cited; but the most striking fact has been the contro-
versy in which the whole subject has been involved' in our times,
arising out of the somewhat startling ideas proposed by Professor
Challis. and so largely discussed by that eminent mathematician and


The first experimental researches of Fresnel do not
date earlier than the beginning of 1815 ; but setting out
from this epoch, memoirs succeeded to memoirs, discov-
eries to discoveries, with a rapidity of which the history
of science offers few examples. On the 28th of December,
1814, Fresnel wrote from Nyons, " I do not know what
is meant by the polarization of light ; beg my uncle, M.
A. Merimee, to send me the best works from which I
may obtain information on this subject." Eight months
had scarcely elapsed, when highly skilful researches
placed him among the most celebrated physicists of our
era. In 1819 he carried off the prize proposed by the
Academy on the difficult question of diffraction. In
1823 he became a member of that body by an una-
nimity of suffrages, — a kind of success extremely rare',
since it implies not only merit of the highest order, but
also, on the part of all the competitors, a frank and ex-
plicit avowal of inferiority. In 1825 the Royal Society
of London admitted him a foreign associate ; and, lastly,
two years later, the same body adjudged to him the
Rumford Medal. This homage from one of the most
illustrious scientific bodies in Europe, — this judgment,
pronounced among a rival people, by the countrymen of
Newton, in favour of an experimenter who attached little
value to his discoveries, except as subverting a system of
which that great genius was the defender, — appears to
me to possess all the characters of a decree which pos-
terity will confirm. I hope, then, it will be permitted
me to appeal to this decree, if in spite of all my desire

Professor Stokes. (See Pkilos. Mag. 1845-6.) We merely allude
to these points in order to show how interesting it would have been
to have become acquainted with the view taken of such a subject by
a mind so eminently anticipative as that of Fresnel. — Translator.



to confine myself to the strict boundaries of truth, and
the consciousness which I have of never having trans-
gressed them, it should happen that this eloge should be
accused of some exaggeration. Though I must avow it
would be a reproach for which I should feel little as the
friend of Fresnel, if it were incumbent on me to repel
it, it would be solely in the capacity of the organ of the
Academy : the office which I this day fill, in the name
of my colleagues, ought to be marked by a precision
and severity as great as that of the exact sciences with
which it is concerned.


The labours of Fresnel almost exclusively relate to
optics. In order to avoid tedious repetitions, I shall
classify them, without regard to the order of dates, in
such a way as to collect in a single group all those which
relate to analogous subjects. The first which will engage
my attention are the phenomena of refraction.

A straight rod partly immersed in water appears bent
or broken ; the rays by which we see the part immersed
must, therefore, have changed their route or have been
broken themselves, in passing out of the water into the
air. It was till lately supposed that to this one observa-
tion we were to restrict the entire knowledge of the an-
cients on the subject of refraction. But in exhuming
from the dust of libraries, where so many treasures are
yet concealed, a manuscript of the optics of Ptolemy, it
has been found that the School of Alexandria had not
confined itself to establishing the mere fact of refraction ;
for this work includes from all incidences, numerical de-
terminations, tolerably exact, of the deviations of the
rays, whether they pass out of air into water or into


glass, or whether they enter glass on passing out of
water. As to the mathematical law of these deviations,
which the Arabian Alhasen, the Pole Vitellio, Kepler,
and other physicists had sought in vain, it is to Descartes
that we owe its announcement : I say Descartes, and
Descartes * alone ; for if the later claims put forth by

* In thus strongly claiming for Descartes the discovery of the law
of refraction which English writers ascribe to Willebrod Snell, Arago
might be supposed actuated by a feeling of national pride, which not,
unfrequently, perhaps, influenced him on questions of this kind. The
strong expression with which he concludes the sentence, seems, how-
ever, to point to a more philosophical motive, and to refer the claim
of Descartes to considerations derived from the connection of the law
of refraction with his theories. However this may be, it may be well
briefly to recapitulate the facts of the case. The ancients, especially
Ptolemy, had amassed many measured results. Alhasen (a.d. 1100)
stated the general principle that refraction in a denser medium causes
the ray to deviate nearer to the perpendicular. Vitellio collected a
number of measured results in different media at different angles of
incidence; among which Kepler attempted, with his usual ardour, to
endeavour to deduce some general numerical relation. He, however,
could proceed no further than this — that while the an(ile of incidence
is but small, it is in a constant ratio (dependent on the nature of the
medium; to that of refraction ; but that, as we deviate more from the
perpendicular, the rule becomes less accurate, and soon fails.

Willebrod Snell, in 1621, investigated and established, by com-
parison of numerical results, a general geometrical mode of repre-
senting the case, which, expressed in modern terms, is the true law of
refraction (or sin i=fJ- sin »•), a constant ratio between the sines, not
the angles, where i and r are the angles of incidence and refraction,
and /J, the constant or refractive index. And the relation observed
by Kepler, which is true so long as the angle is small enough to be
nearly proportional in its sine, is thus extended and generalized.
Snell died in 1626 without having printed his discovery; but it had
been shown in MS. to many persons, especially to Huyghens, who
fully perceived its value and importance. And it is on his authority
that the discovery was properly assigned to Snell by Montucia,
Bossut, and other writers. Huyghens, however, did not publish any
account of the matter till it appeared in his Dioptricu, which was
printed after his death in 1700.



Huyghens in favour of his fellow countryman Snell be
accepted, we must give up the pretence of writing the
history of science.

A mathematical law has more importance than an
ordinary discovery, for it is itself a source of discoveries.
From it simple analytical transformations point out to
observers a multitude of results more or less hidden, of
which they would with difficulty have become aware ;
but such results cannot be accepted without reservation,
so long as the truth of the primary law rests solely on
measurements. It is necessary for science that this law
should acquire that character of demonstration which
mere experiments alone, however precise, cannot confer,
by being traceable upwards to the first principles of

Descartes then attempted to establish his law of re-
fraction by considerations purely mathematical ; perhaps
it was thus also that he discovered it? Fermat com-
bated the demonstration of his rival, and replaced it by
a method more rigorous, but which had the serious fault

Vossius states that, among others, the contents of Snail's MS. were
shown to Descartes.

That philosopher, however, in a manner very usual with him, com-
mences treating the subject on entirely original grounds; and, in the
course of a purely theoretical speculation deduces the same law of
refraction as a consequence of his a priori principles (DiojHrica, 1637,
ch. il. § 9), without making the slightest allusion to Snell. Hence
the discovery of the law has been assigned to him, especially by
French writers. It is to be observed, however, that he in no waj' at-
tempts to found his deduction on any comparison of experimental
results. Thus, even admitting that Descartes is entitled to the estab-
lishment of the law as a iheoretical deduction, he clearly has no claim
to the experimental verification of it, which is by far the most material
point; and the more so as his theory is based on the assumption, now
proved to be ftilse, that light is acceleraled in passing through the
denser medium. — Translator.


of being dependent on a metaphysical principle of which
he did not show the necessary truth.* Huyghens ar-

* The theoretical principles here glanced at, are those connected
■with speculations on one of the most curious points presented by the
theory of light; which, perhaps, it may be desirable briefly to explain.
Ptolemy had shown that when light is reflected from any surface, the
law of reflexion, or equality of angles, is precisely that which causes
light to pass from any one point in its course, before incidence to any

Online LibraryF. (François) AragoBiographies of distinguished scientific men (Volume 2) → online text (page 14 of 38)