F. (François) Arago.

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series of greater joint elevations; but if the elevations of one series
are so situated as to correspond to the depressions of the other, they
must exactly fill up those depressions, and the surface of the water
must remain smooth; at least, I can discover no alternative, either
from theory or from experiment. Now, I maintain that similar efiects
take place whenever two portions of light are thus mixed; and this I
call the general law of the interference of light." i — Translator.

For the sake of many readers, it may not be superfluous or useless
here briefly to illustrate the application of these theoretical ideas.
We have only to imagins in like manner, in the case of the rays of
light, two sets of waves propagated through an ethereal medium and
coinciding in direction, when it will be easily apparent that just as in
the case of the supposed canal, they may have their waves either
conspiring or counteracting, and consequently giving a point of bright-
ness or darkness accordingly.

Thus, a coincidence in the periods, or an interval of an integer num-
ber of entire wave-lengths, would cause the two sj'stems of waves to
conspire and reinforce each other; a diflereuce of periods of half a
wave-length, or any odd number of half wave-lengths, would cause
the two systems to counteract or neutralize each other. Thus,
according to the thickness, there would be a point of darkness or of
brightness for each primary ray, and the succession of tints would
be perfectly explained.

This would directlj' apply to the tliinjilms. A ray impinging would
be partly reflected at the first surface of the thin film, partly entering

1 Works, vol. i. p. 202.



INTERFERENCE OF LIGHT.



305



tain point in gpace. At this point was placed a sheet of
white paper. Each ray, taken by itself, made the paper

it would be reflected internally at its second surface, and emerge
coinciding in direction with the first, but retarded behind it from the
thickness traversed in its undulations either by a whole, or half
undulation, or some multiples of these,— thus giving either a point of
brightness or one of darkness accordingly; or by some intermediate
fraction, giving an intermediate shade. And this would go on alter-
nately at successively greater thicknesses of the film, giving a suc-
cession of such points or bands.




Thus at two successive thicknesses of the plate (p), the incident
rays falling on it in parallel directions, i i, are reflected partially from
the first surface, r r, and partially from the second, rl r'. According
to the diflerence of thickness traversed, these may be in accordance
giving a point of brightness as at +, or in discordance giving a point
of darkness as at °.

If two rays or sets of waves, instead of being exactly superimposed
be supposed to meet inclined at a very acute angle, in a somewhat
similar way they would, at a series of points, alternately conspire or
clash with each other, thus giving rise to a series of bright and dark
points, the assemblage of which will pi-oduce bands or stripes on a
screen intercepting the rays. Now as to actual experimental cases,
it was in the application of this latter theoretical idea that the inven-
tion of Dr. Young was peculiarly displayed. The former case was
that alone which seems to have occurred to Hooke in reference to the
colours of thin plates, and even this was in his mind but a very indefi-
nite conception; nor did it seem at first sight readiU' comparable with
such cases as the difl'raction fringes, or still less with the internal
bands of a shadow observed by Grimakli. If Hooke had imagined
anv theoretical views of this kind, it was probably confined to the



30G



THOMAS YOUNG.



more bright at that point, but when the two rays united
and arrived at that point together, all brightness dis-
appeared ; complete night succeeded to day.

one case of the thin films; Young's great merit was the comprehen-
siveness of his principle; and in following out the investigation, he
proceeded at once to such a generalization as evinced that compre-
hensiveness and connected immediately those classes of phenomena
apparently so different in character,— the thin films, the internal
bands, and the external fringes. When, as in Grimaldi's experiment



/-since called the phenomena of diffraction), a narrow slip of card was
placed in a very narrow beam of solar light, dark and bright stripes
parallel to the sides internally -marked the whole shadow longitudi-
nally, while the external fringes appeared on the outside at each edge.
The general appearance of the shadow of a long narrow body with
parallel sides in a beam of solar light issuing from a minute hole in a
shutter, or, what is better, the focus of a small lens collecting the
rays to a point, is that of a shadow marked witli longitudinal stripes
and externally bordered by parallel fringes or bands of liglit slightly
coloured, as seen in the annexed figure.



DIFFRACTION.



307



Two rays do not always annihilate each other com-
pletely at their point of intersection. Sometimes we

To exhibit these appearances ordinarily requires the sun's light.
But the translator has found a very simple method of exhibiting these
phenomena on a minute scale by candle light, by merely placing a
fine wire across one surface of a lens of short focus, and loolting
through it at light admitted through a narrow slit parallel to the wire,
or even the flame of a candle at a considerable distance.

Next, as to the theoretical explanation, an inspection of the accom-
panying diagram will perhaps help to convey an idea of the manner
in which the sevei'al sets of waves are formed and iuterfere in the case
now supposed.

Young conceived the beam of light as a series of waves propagated
onward, till, on reaching the card, they were broken up into two new
sets of waves spreading in circles round each edge as a new centre,
while part of the original set continued to pass on at each side. On
the principle just mentioned these would interfere with the new por-
tions on the outside; and the two new portions would interfere with




308



THOMAS YOUNG.



observe only a partial weakening of intensity, sometimes,
on the other hand, the rays conspire and increase the
illumination. Every thing depends on the difference in
the length of route which they have gone through, and




each other in the inside of the shadow; In either case giving stripes
or bands. To complete the proof, when an opaque screen was phiced
so as to intercept the rays on one side, though abundance of light was
present on the other, yet all the internal bands immediately disap-
peared; demonstrating that the effect was due solely to the concur-
rence of the light from both sides. The bands produced by light
admitted through narrow apertures, and numerous other phenomena
of the same kind, may receive a general and popular explanation in
the same way. — Translator.



PERIODICAL COLOURS. 309

that, according to very simple laws, the discovery of
which in any age would suffice to immortalize a physi-
cist.

The differences of route which produce these conflicts
between the rays, accompanied hy their entire mutual
destruction, have not the same numerical value for the
differently coloured primary rays. When two white rays
cross, it is then possible that one of their chief constituent
parts, tlie red, for example, may alone be in the condition
fit for mutual destruction. But white, deprived of its
red, becomes green ! Thus intei'ference of light mani-
fests itself in the phenomena of coloration. Thus the
different elementary colours are placed in evidence with-
out any prism to sei)arate them. We should, however,
remark that there does not exist a single point in space
where a thousand rays of the same origin do not proceed
to cross one another after reflexions more or less oblique,
and we shall perceive at a glance the whole extent of
the unexplored region which interferences open to the
investigations of experimenters.

When Young published this theory, many phenomena
of periodical colours had been already ofi"ered to the no-
tice of observers ; and we should add, had resisted all
attempts at explanation. Among the number we mio-ht
instance the coloured rings which are formed by reflexion,
not on thin films, but on mirrors of thick glass slightly
concave ; the irridescent bands of different breadths with
which the shadows of bodies are bordered on the outside,
and in some instances covered within, which Grimaldi
first noticed, and which afterwards uselessly exercised
the genius of Newton, and of which the completion of
the theory was reserved for Fresnel ; the bows coloured
red and green, which are perceived in greater or less



310 THOMAS YOUNG.

number immediately under the innermost of the prismatic
bands of the rainbow,* and wliich seemed so completely
inexplicable, that the writers of elementary books on
physics had given up making mention of them ; and lastly,
the "coronas," or broad coloured circles with varying
diameters, which often appear surrounding the sun and
moon.

If I call to mind how many persons do not appreciate
scientific theories, except in proportion to the immediate
applications which they may otfei-, I cannot terminate
this enumeration of the phenomena which characterize
the several series of more or less numerous periodical
colours, Avithout mentioning the rings, so remarkable by
their regularity of form and purity of tint with which
every brilliant light appears surrounded, when we look
at it through a mass of fine molecules or filaments of
equal dimensions. These rings, in fact, suggested to
Young the idea of an instrument, extremely simple,
which he called an " eriometer," and with which we
can measure without difficulty the dimensions of the most
minute bodies. The eriometer, as yet so little known to
observers, has an immense advantage over the mici'o-
scope in giving at a single glance the mean magnitude of
millions of particles which are contained in the field of
view. It possesses, moreover, the singular property of
remaining silent when the particles differ much in mag-
nitude among themselves, or, in other words, when the
question of determining their dimensions has no real
meaning.

Young applied his eriometer to the measurement of
the globules of blood in different classes of animals, — to

* This explanation has been recently controverted by Professor
Potter.— P/ij7os. Mag. May, 1855.



ERIOMETER. 311

that of powders furnished by different species of vegeta-
bles, of the fineness of different kinds of fur used in the
manufacture of different fabrics, from tliat of tlie beaver,
the most valuable of all, down to that of the common
sheep of the Sussex breed, which stands at the other
extremity of the scale, and is composed of filaments four
times and a half thicker than that of the beaver.

Before the researches of Young, the numerous phe-
nomena of colours* Avhich I have just pointed out were
not only inexplicable, but nothing had been found to
connect them with each other. Newton, who was long
engaged on the subject, had not perceived any connection
between the rings in thin films and the bands of diffrac-
tion. Young reduced these two kinds of coloured bands
alike to the law of interference. At a later period, when
the coloured phenomena of polarization had been discov-
ered, he observed, in certain measures of the thicknesses
at which they occurred, some remarkable numerical analo-
gies, Avhich made it very reasonable to expect that sooner
or later this singular kind of polarization would be found
connected with his doctrine. He had in this instance,
however, we must admit, a very wide hiatus to fill up.
The knowledge of some important properties of light,
then completely unknown, would have been necessary to
permit him to conceive the whole singularity of the effects
which in certain crystals, cut in certain directions, double

* Every one may have remarked the threads of a spider's web occa-
sionally exhibiting brilliant colours in the sunshine. The same thing
is seen in fine scratches on the surface of polished metal, produced in
a more regular way, by the fine engraved parallel grooves in Barton's
buttons. The colours of mother-of-pearl are of the same kind; all
these colours Dr. Young showed were due to interference of the por-
tions of light reflected from the sides of the narrow transparent thread
or groove. — Translator.



312 THOMAS YOUNG.

refraction produces by the destruction of light resuUiiig
from the interference of rays ; but it is to Young that the
honour belongs of having opened the way ; it was he
who was the first to decypher these hieroglyphics of
optics.*

* It has been well observed that simplicity is not always a fruit
of the first growth, and accordingly some of the earliest of Young's
researches were complicated by unnecessary conditions. Thus, to
exhibit the effect of two rays interfering, he at first not unnaturally
transmitted the narrow beam of light through two small apertures near
together. In point of fact, though the real effect is liere seen, it is
mixed up with others of a more complex kind. The narrow apertures
each exhibited coloured fringes, in addition to the interference stripes
seen between them. The coloured fringes of apertures (unless very
wide) are distinct from those formed by one external edge of an 9paque
body ; the light from each side conspires to the effect in a somewhat
complex manner. If the aperture be otherwise than long with parallel
sides, the phenomenon becomes still more complex, and the calcula-
tion difficult; few such cases have ever yet been solved, and some such
cases have been dwelt upon as formidable objections to the theory;
they are simply cases to which the formula, from its mathematical
difficulties, has not yet been extended.

In all these cases of diffraction an opaque body was used, and it
might still be suspected that some action of the edge of that body might
be concerned in the result. Numerous experiments of Maraldi, Dutour,
Biot, and others, were directed to the investigation of this point. Biot
showed that an opaque body was not necessary, inasmuch as the edge
of a plate of glass, or even the bounding line of two fiices of a glass cut
at a slight inclination to each other, gave the same fringes; indeed,
Newton also had noticed something of the kind. Haldat varied the
conditions of the edge in every conceivable way, whether of form or
nature, by the influence of magnetism, galvanism, electricity, or tem-
perature from freezing to a red heat, without producing the slightest
difference in the fringes; a result which it would be impossible to con-
ceive compatible with any idea of an atmosphere of attraction or repul-
sion surrounding the edge.

Again, though we have given the explanation of the external fringes
in its simple and correct form, yet both Young and Fresnel failed in
the first instance to see it in that light, both believing that the reflexion
of a portion of rays from the edge of the opaque body was mainly con-
cerned in producing the interference. Subsequent experiments showed



HIEROGLYPHICS. 313



EGYPTIAN HIEROGLYPHICS. HISTORY OF THE FIRST

EXACT INTERPRETATION GIVEN OF THEM.

The word hieroglyphic, regarded not metaphysically,
but in its natural acceptation, carries us into a field which

that even in cases where that edge reflects any sensible amount of
light, its influence on the diftVacted fringes is quite inappreciable. In
fact, Young, in a letter to Fresnel, in returning thanks for a copy of a
later memoir in which he had shown this suiiposition to be unneces-
sarj', also concui's in abandoning it. It did but complicate and injure
the beauty of the result. ^ And every doubt must have disappeared in
the minds of those who compared the minute arithmetical accuracy
with which the places of the fringes, as computed from the simple
theory in the investigations of Fresnel, agreed with those actually de-
termined by the nicest micrometical measurements.

In enumerating the discoveries of Young in the first establishment
of the wave theory, it is somewhat singular that Arago (whether from
accident or design) should have overlooked one investigation which
must be regarded as among the most important. The great support
which the emission theory received in recent times was that derived
from Laplace's memoir on the law of double refraction (1809), in
which, on the principle of "least action," as maintained by Mauper-
tuis and applied to the idea of luminous molecules, he explained the
observed laws of ordinary and extraordinary refraction in Iceland spar.
This investigation exercised a powerful influence in favor of the mole-
cular theory over the minds of the men of science in France who bowed
implicitly to the authority of Laplace. But the memoir of Laplace was
the subject of a very powerful attack on the part of Dr. Young, carried
on in an article in the Quarterly lieview, November, 1809, in which he
disputed the mechanical and mathematical grounds of Laplace's the-
ory, and showed that the same laws of double refraction could be far
more easily deduced from the unduiatory hypothesis. Next to the
discovery of interference, this refutation of the strongest point of the
emission theory cannot but be regarded as one of the most material in
the development and establishment of the unduiatory view.

To the statement of these various cases of interference it should be
added that when the tints of polarized light were discovered. Young
in 1814, applied to the phenomena the f/encrcd consideration of iwie?'-

1 Young's Works, i. 393.

BEC. 8ER. 14



314 THOMAS YOUNG.

has been long the theatre of numerous and animated de-
bates. I have hesitated whether to risk offending the
feelings which tliis question has excited. The secretary
of an Academy occupied exclusively with the exact
sciences, might indeed, without impropriety, remit this
philological subject to other more competent judges. I
also feared, I will avow, to find myself in disagreement
on several important points with the illustrious man of
science whose labours it has been so delightful for me to
analyze, without having to add a word of criticism from
my pen. All these scruples, however, vanish when I re-
flect that the interpretation of hieroglyphics has been one
of the most beautiful discoveries of our age ; that Young
himself has mixed up my name with discussions relating
to it ; that to examine whether France can jiretend to
this new title to glory, is to enhance the importance of
the task, confided to me at this moment, and to perform
the duty of a good citizen. I am aware that some may
find narrowness in these sentiments. I am not ignorant
that the cosmopolitan spirit has its good side ; but with
what name shall I stigmatize it, if, when all neighbouring

ference, that is to say, he showed that owing to the differing obliquities
of the paths of the rays within the crystal they would be unequally
retarded in their passage, and would consequently emerge in condi-
tions, with regard to length of route, respectively of accordance or dis-
cordance at corresponding distances round the centre line or axis of
the crystal, and thus might give rise to coloured rings. Arago, how-
ever, soon noticed that the explanation was incomplete ; the main point
in fact remained to be accounted for, viz: why we see no colours till
the analyzer is applied, and why even the previous polarization is
necessary to the result. It was not until about two years afterwards
that Arago and Fresnel jointly succeeded in discovering a new law,
which not only furnished the complete solution of the polarized rings,
but at length cleared away all the difficulties which from the first had
surrounded the idea of polarization itself For an account of this .-iee
memoir of Fresnel. — Trandatar.



UIEROGLYPHICS. 315

nations enumerate with triumph the discoveries of their
sons, it should hinder me from seeking, even in the pres-
ent circle, among those colleagues whose modesty I would
not hurt, the proof that France is not degenerate ; that
she also adds every year her glorious contingent to the
vast deposit of human knowledge.*

I approach, then, the question of Egyptian writing,
and I do so free from all prejudice, with the firm wish
of being just ; with the lively desire to conciliate the
rival pretensions of two men of science whose premature
death has been to all Europe a legitimate subject of re-
gret. Lastly, I shall not in this discussion on hieroglyphics
transgress the bounds imposed on me ; happy if those
who listen to me, and whose indulgence I ask, may find
that I have known how to escape the influence of a sub-
ject whose obscurity is proverbial.

Men have imagined two systems of writing entirely
distinct. One is that employed by the Chinese, which is
the system of hieroglyphics ; the other, at present in use
among all other nations, bears the name of the alphabet-
ical or phonetic system.

The Chinese have no letters projierly so called : the
characters which they use in writing are strictly hiero-

* In bringing out a part of this chapter on Egyptian Hieroglyphics
in the Annuaire for 1836, Arago has added: " Tlie first exact inter-
pretation which has been given of Egyptian hieroglyphics will cer-
tainly talce its place among the most beautiful discoveries of the age.
Besides, after the animated debates to which it has given birth, every
one would desire to know whether France can conscientiously pretend
to this new title to glory. Thus the importance of the question, and
the national self-love properly understood, unite in encouraging me to
publish the result of a minute examination to which I have devoted
myself. Can I, then, be blind to the danger which there always is in
attempting difficult subjects in matters which we have not made the
special subject of our studies ? "



316 THOMAS YOUNG.

glyphics ; they do not represent soutids or articulations,
but idecis. Thus a house is represented by a unique and
special character, which does not change even when the
Chinese have come to call a house, in their spoken lan-
guage, by a nanae totally different from that which they
formerly pronounced. Does this result appear surpris-
ing ? Imagine the case of our cyphers, which are also
hieroglyphics ; the idea of one added to itself seven times
is expressed everywhere in France, in England, in Spain,
&c., by the aid of two circles placed vertically one over
the other, and touching in one point ; but in looking at
this hieroglyphic sign (8) the Frenchman pronounces
" huit," the Englishman " eight," the Spaniard " ocho."
No one is ignorant that it is the same with compound
numbers. Thus, to speak briefly, if the Chinese idio-
graphic signs were generally adopted, as the Arabic
numerals are, every one would read in his own language
the works which they presented to him, without the need
of knowing a single word of the language spoken by the
authors who have written them.

It is not so with alphabetical writing : —

" He who first taught us the ingenious art
To paint our words, and speak them to our eyes,"

having made the capital remark that all words of a spoken
language, even the most rich, are compounded of a very
limited number of elementary articulate sounds, invented
artificial signs or letters to the number of twenty-four
or thirty to represent them. By the aid of these signs
differently combined he could write every word which
struck his ear even without knowing the meaning
of it.

The Chinese or hieroglyphic writing seems to be the
infancy of the art. It is not always, as has been some-



HIEROGLYPHICS. 317

times said, that to learn to read it, even in China, occu-
pies the whole life of a studious Mandarin. Rerausat
(whose name I cannot mention without recalling one of
the most heavy losses which literature has lately sus-
tained) has established, both by his own experience and
by the fact of the excellent scholars he has formed every
year by his lectures, that we may learn Chinese like any
other language. It is not true, as was once imagined,
that the characters are appropriated solely to the expres-
sion of common ideas ; several pages of the romance of
Yu-kiao-li, or the Two Cousins, will suffice to show that
the most subtle abstractions, the quintessence of refine-
ments, are not beyond the range of the Chinese writing.
The chief fault of this mode of writing is, that it gives no
means of expressing new names. A letter from Canton
might have told at Pekin, that on the 14th of June, 1800,



Online LibraryF. (François) AragoBiographies of distinguished scientific men (Volume 2) → online text (page 24 of 38)