F. (François) Arago.

Biographies of distinguished scientific men (Volume 2) online

. (page 23 of 38)
Online LibraryF. (François) AragoBiographies of distinguished scientific men (Volume 2) → online text (page 23 of 38)
Font size
QR-code for this ebook


of the functions of the different media of which the eye
is composed ; and showed themselves contented, as were
at a later period Galileo and Newton, not to ascend
above those kinds of knowledge which are capable of
being elaborated or corrected by the aid of our senses,
and which had been stigmatized under the porticos of
the Academy by the contemptuous epithet of simple
opinion. Such is always human weakness that, after
having followed with a rare success the principal devia-
tions which light undergoes in passing through the cornea
and the crystalline, Maurolycus and Porta, when very
near attaining their object, stopped short, as if before an
insurmountable difficulty, when it was objected to their
theory that objects ought to appear in an inverted posi-
tion if the images formed in the eye are themselves
inverted. The adventurous spirit of Kepler, on the
contrary, did not remain embarrassed. It was from
psychology that the attack originated ; it was equally
from psychology — clear, precise, and mathematical — that
he overthrew the objection. Under the powerful hand
of this great man, the eye became, definitively, the
simple optical apparatus known by the name of the



THEORY OF THE EYE. 293

camera obscura ; the retina is the ground of the picture,
the crystalline replaces the glass lens.*

This assimilation, generally adopted since Kepler's
time, remains open only to one diificulty ; the camera
obscura, like an ordinary telescope, requires to be brought
to a projjer focus according to the distance of objects.
When objects are near it is indispensable to increase the
distance of the picture from the lens ; a contrary move-
ment becomes necessary as they become more distant.
To preserve to the images all the distinctness which is
desirable, without changing the position of the surface
which receives them, is therefore impossible : at least,
always supposing the curvature of the lens to remain
invariable ; that it cannot increase when we look at near
objects, or diminish for distant objects.

* The author seems to have left this illustration incomplete. Kep-
ler's suggestion of the identity of the eye with the camera obscura,
after all, does not touch the difficulty of the inversion of the image.
Nor has it been considered as completely cleared up even till much
later times. The solution which, it is believed, is now most generally
assented to is this. It is a law of our constitution, dependent on some
physiological principle unknown, that we refer impressions on the
retina to objects existing, or believed to exist, in the rectilinear direc-
tion fi-om which the impression comes to tlie retina. Consequently,
as rays cross at the pupil, an impression arriving at (r) in the direction




of the arrow, will convey the idea of an object existing at (5') ; in other
words, a ray falling on the upper part of the retina suggests an object
Ij'ing bekiw, or an inverted image suggests an ei^ect object.



294 THOMAS YOUNU.

Among the different modes of obtaining distinct images,
nature has assuredly made a choice, since man can see
with great distinctness at very different distances. The
question thus put has afforded a wide subject of remark
and discussion to physicists, and great names have figured
in the debate.

Kepler and Descartes held that the whole ball of the
eye is susceptible of being elongated and flattened.

Porterfield and Zinn contended that the crystalline
lens was movable ; and that it could place itself nearer
to, or further from the retina, as might be needed.

Jurin and Musschenbroeck believed in a change in the
curvature of the cornea.

Sauvages and Bourdelot supposed also that a change
in curvature took place, but only in the crystalline lens.
Such is also the system of Young. Two memoirs which
our colleague successively submitted to the Royal Society
of London include the complete development of his views.

In the first of these, the question is treated almost
entirely in an anatomical point of view. Young there
demonstrates by the aid of direct observations of a very
delicate kind, that the crystalline is endowed with a
fibrous or muscular constitution, admirably adapted to
all sorts of changes of form. This discovery overthrew
the only solid objection which had, till then, opposed the
hypothesis of Sauvages and Bourdelot.

That hypothesis had no sooner been announced than it
had been attacked by Hunter.

Thus this celebrated anatomist aided the cause of the
young experimenter by the attention drawn to the sub-
ject, while his labours were as yet unpublislied, and not
even communicated to any one. Howevei", this point of
the discussion soon lost its importance. The learned



THKORY OF THE EYE. 295

Leuwenlioeck, armed with his powerful microscopes,
traced out, and gave figures of, the muscular fibres in
all their ramitications ia the crystalline of a fish. To
awaken the attention of the scientific world, tired with
these long debates, nothing less was necessary than the
high renown of two new members of the Royal Society
who entered the lists : one, a celebrated anatomist, the
other the most eminent instrument-maker of whom Eng-
land could boast. These jointly presented to the Royal
Society a memoir, the fruit of their combined labours,
intended to establish the complete unalterability of the
form of the crystalline. The scientific world was not
prepared to admit that Sir Everard Home and Ramsden
together, could possibly make inaccurate experiments, or
be deceived in micrometical measurements. Young him-
self could not believe it ; and in consequence he did not
hesitate publicly to renounce his theory.

This readiness to own himself vanquished, so rare in
a young man of twenty-five, and especially on the occa-
sion of a first publication, was in this instance an act of
modesty without example. Young, however, had really
nothing to retract. In 1800, after having withdraAvn
his former disavowal, our colleague developed anew the
theory of the change of form of the crystalline in a me-
moir against which, from that time, no serious objection
has been brought.

Nothing could be more simple than his line of aro-u-
ment ; nothing more ingenious than his experiments.
Young, in the first instance, got rid of the hypothesis
of a change of curvature in the cornea by the aid of
microscopic observations, which were of a kind to ren-
der the most minute variations appreciable. We can
say more ; he placed the eye in special conditions where
changes of curvature in the cornea would have been



296 THOMAS YOUNG.

■without eifect ; he phinged the eye in water, and proved
that there was still the same faculty of seeing at different
distances perfectly preserved. Tlie second of three pos-
sible suppositions, that of an alteration in the dimensions
of the whole organ, was again overthrown by a multitude
of objections and of experiments which it was difficult to
resist.

The problem thus seemed finally settled. Who does
not see, in fact, that if, of three only possible solutions,
two are put out of the question, the third is necessarily
established ; that if the radius of curvature of the cornea
and the longitudinal diameter of the wJwIe eye are inva-
riable, it must follow that the form of the crystalline is
invariable ? Young, however, did not stop there ; he
proved directly, by the minute phenomena of the changes
in the images, that the crystalline really changes its cui'-
vature ; he invented, or at least, gave perfection to, an
instrument susceptible of being employed even by the
least intelligent persons, and those least accustomed to
delicate experiments ; and, armed with this new means
of investigation, he assured himself that those individuals
in whose eyes the crystalline has been removed in the
operation for cataract, did not enjoy the faculty of seeing
equally distinctly at all distances.*

* This instrument, called an " Optometer," was originally proposed
by Dr. Porterfield, and consists of a simple and ingenious contrivance
for ascertaining the focal length of the eye, which varies so greatly in
different individuals, and often in two eyes of the same person, and in
the same eye under different conditions. Dr. Young greatly improved
upon the original construction. It will be found described in the Lec-
tures on Natural Philosopluj, vol. ii. p. 576. The principle of it consists
in measuring accurately the distance of an object from the eye at
which perfectly distinct vision is obtained, and which is determined
when the object, seen through two small apertures close to the eye,
presents only a single image, while in other positions it shows two
images. — Translator.



VISION AT DIFFERENT DISTANCES. 297

We might fairly be astonished that this admirable the-
ory of vision, this combination so well framed when the
most ingenious reasonings and experiments lent each
other mutual support, did not occupy that distinguished
rank in the science of the country which it deserved.
But to explain this anomaly, must we necessarily recur
to a sort of fatality ? Was Young then really, as he
sometimes described himself with vexation, a new Cas-
sandra, proclaiming incessantly important truths which
his ungrateful contemporaries refused to receive ? We
should be less poetical, but more true, it seems to me, if
we remarked that the discoveries of Young were not
known to the majority of those who would have been
able to appreciate them. The physiologists did not read
his able memoir, because in it he presumes upon more
mathematical knowledge than is usually attained in that
branch.

The physicists neglected it in their turn, because in
oral lectures, or printed works, the public demands little
more at the present day than superficial notions, which
an ordinary mind can penetrate without difficulty. In all
this, whatever our distinguished colleague may have be-
lieved, we perceive nothing out of the ordinary course.
Like all those who sound the greatest depths of science,
he was misunderstood by the multitude ; but the applauses
of some of the select few ought to have recompensed him.
In such a question we ought not to count the suffrages ; —
it is more wise to tveigh them.*

* Arago, in assigning tlie probable causes of the neglect of Young's
speculations, seems to fall short of his usual point and perspicuity. It
might be true that his memoir was neglected by physiologists because
it was mathematical, and by parity of reason it might have been neg-
lected by physicists and mathematicians as being physiological. But
it is surely no reason to say that it was neglected by physicists because
l.S *



298 THOMAS YOUNG.



INTERFERENCES.

The most beautiful discovery of Young, that which will
render his name imperishable, was suggested to him by
an object in appearance very trivial ; by those soap bub-

the public are superficial, &c. Young may have been, in most of his
speculations, too profound for the many; but this particular instance
of the structure of the eye and theory of vision is, perhaps, of all his
researches, that which can be the least open to this charge. The sub-
ject is not itself abstruse: it is one easily understood bj' every edu-
cated person, without mathematical attainments ; and the point at
issue was a simple question of fact requiring no profound physiological
knowledge to appreciate, whether the crystalline has or has not a mus-
cular structure capable of changing its convexity. The real state of
the case seems to be very satisfactorially explained by Dean Peacock
(p. 36, et seq.), from whose account, as well as from what has been since
written, it appears, after all that has been done both by Dr. Young and
others, that there is even at the present day considerable difference of
opinion on the subject.

Perhaps the most comprehensive survey of the whole subject which
recent investigation has produced will be found in the paper of Pro-
fessor J. D. Forbes in the jE/lin. Traiisat turns, vol. xvi. part I. 1845.
After giving a summarj' view of preceding researches, and adverting
to the prevalent opinion atnong men of science, that the true explana-
tion yet remains to be discovered (most anatomists den3'ing as a fact
the existence of the muscular structure which Young conceived he had
proved). Professor Forbes proposes, as his own view of the cause, the
consideration of the remarkable varialiuii in density of the crystalline
towards its central part; coats of different density being disposed in
different layers, may be acted ou by the pressure of the humours of
the eye when the external action of the muscles compresses them, and
thus increase tlie curvature of the lens, when the eye is directed to a
near object, the whole consistence especially in the outer parts being
of a gelatinous or compressible nature, and the central part more solid
and more convex. Thus uniform pressure on the outer parts would
tend to make the outer parts conform more nearly to the more convex
interior nucleus.

It may be added that mtiiiy physiologists are of opinion that, after all,
there does not exist a sufficient compressive action on the ball of the
eye to produce the effect supposed. — Trandatur.



INTERFERENCES. 299

bles so brilliantly coloured, so light, wliicli when just
blown out of a pipe become the sport of every impercep-
tible current of air. Before so enlightened an audience,
it would without doubt be superfluous to remark that the
difficulty of producing a phenomenon, its variety, its util-
ity to the arts, are not the necessary indications of its
importance in a scientific point of view. I have, there-
fore, to connect with a child's sport the discovery which
I proceed to analyze, with the certainty that its credit
will not suffer from its origin. At any rate I shall have
no need to recall the apple, which, dropping from its stalk
and falling unexpectedly at the feet of Newton, developed
the ideas of that great man respecting the simple and
comprehensive laws which regulate the celestial motions ;
nor the frog and the touch of the bistoury, to which phys-
ical science has recently been indebted for the marvellous
pile of Volta. Without referring in particular to soap
bubbles, I will suppose that a physicist has taken for the
subject of experiment some distilled water, that is to say,
a liquid, which in its state of pui'ity never shows any
more than some very slight shade of colour, blue or green,
hardly sensible, and thaj; only when the light traverses
great thicknesses. I would next ask what we should
think of his veracity if he were to announce to us, with-
out further explanation, that to this water, so limpid, he
could at pleasure communicate the most resplendent col-
ours ; that he knew how to make it violet, blue, green ;
then yellow like the peel of citron, or red of a scarlet tint,
without affecting its purity, without mixing with it any
foreign substance, Avithout changing the proportions of its
constituq^it gaseous elements. Would not the public re-
gard our physicist as unworthy of all belief, especially
when, after such strange assertions, he should add, that



300 THOMAS YOUNG.

to produce colour in water, it suffices to reduce it to the
state of a thin fihu ; tliat ''thin" is, so to speak, the syn-
onym of "coloured ; " that the passage of each tint into
one the most different from it is the necessary conse-
quence of a simple variation of the thickness of the liquid
film ; that this variation, for instance, in passing from red
to green, is not the thousandth part of the thickness of a
hair ! Yet these incredible propositions are only the
necessary consequences deduced from the accidental ob-
servation of the colours presented by soap bubbles, and
even by extremely thin films of all sorts of substances.

To comprehend how such phenomena have, during
more than 2000 years, daily met the eyes of philoso-
phers without exciting their attention, we have need to
recollect to how few persons nature imparts the valuable
faculty of being astonished to any purpose.

Boyle was the first to penetrate into this rich mine.
He confined himself, however, to the minute description
of the varied circumstances which gave rise to these
iridescent colours. Hooke, his fellow-labourer, went fur-
ther. He believed that he had discovered the cause of
this kind of colours in the coincidences of the rays, or to
speak in his own language, in the mutual action on each
other of the waves reflected by the two surfaces of the
thin film. This was, we may admit, a suggestion char-
acteristic of genius ; but it could not be made use of at
an epoch when the compound nature of white light was
not as yet understood.

Newton made the colours of thin films a favourite
object of study. He devoted to them an entire book of
his celebrated treatise the " Optics." He established
the laws of their formation by an admirably connected
chain of experiments, whi(;h no one has since surpassed



COLOURS OF THIN FILMS. 301

in excellence. In illuminating with homogeneous light
the very regularly formed series of bands of which
Hooke had already made mention, and which originated
round the point of contact of two lenses pressed closely
together, he proved that for each species of simple
colour there exists, in thin films of every substance, a
series of thicknesses gradually increasing, at each of
which no light is reflected from the film. This result
was of capital importance ; it included the key to all
these phenomena.

Newton was less happy in the theoretical views which
these remarkable observations suggested to him. To
say, with him, that the luminous ray which is reflected
is "in a fit of easy reflexion," — to say that the ray
which passes through the film entire, is " in a fit of easy
transmission," — what is it but to announce, in obscure
terms, merely the same fact which the experiment with
the two lenses has already taught us ? *

* In regard to the theory of the " fits," the author here seems to
represent Newton's view, as in fact mere tautology; while in other
places he is supposed to have indulged in a visionary theory on the
subject. Newton, however, expressly says, " what kind of action or
disposition this is ; — whether it consist in a circulating or vibrating
motion of the ray, or of the medium, or something else, I do not here
inquire." (Optics, p. 255, ed. 1721.)

The fact is, Newton in his optical researches expressed the same
avowed and systematic dislike to indulging in a7iy gratuitous theories
as in his other inquiries. " Hypotheses non fingo " was his motto in
these as well as other researches. In adopting the idea of " fits of
easy reflexion and transmission," we are of opinion that he did not
violate that maxim, and that it was in fact the only legitimate first
expression of the conclusion which the facts warranted. At certain
points no light appeared; it was the legitimate inference, in the then
state of knowledge, that none was reflected. But light was clearly
under the same circumstances transmitted ; at a distance a little
greater along the ray, an opposite effect was witnessed; and so on.



302 THOMAS YOUNG.

The theory of Thomas Young is not amenable to this
criticism. Here there is no longer admitted any peculiar
kind of " fits " as primordial properties of the rays.
The thin film is here assimilated in all respects to any
thicker reflector of the same substance. If at certain
points in its surface no light is visible, Young did not
conclude that therefore its reflexion had ceased ; he sup-
posed tliat, in the special directions of those points, the
rays reflected by the second surface proceeded to meet
with those reflected from the first surface, and com-
pletely destroyed them. This conflict of the rays is
what the author designated by the term " interference^
which has since become so famous.

Observe then here the most singular of hypotlieses !
We must certainly feel surprised at finding night in full
sunshine, at points where the rays of that luminary
ari'ive freely ; but who would have imagined that we
should thence come to suppose that darkness could be
engendered by adding light to light !

A physicist is truly eminent when he is able to an-
nounce any result which, to such an extent, clashes with
all received ideas ; but he ought, without delay, to sup-
port his views by demonstrative proofs, under the pen-
alty of being assimilated to those Oriental writers whose
fantastic reveries charmed the thousand and one nights
of the Sultan Schahriar.

It was nothing more than the strict inference that at those points suc-
cessively something occurred in the course of the ray which disposed it
for, or induced, reflexion in tlie one case, and non-reflexion in the
other; accompanied in the latter case by the like tendency to trans-
misi^ion. These apparent "fits" must be still acknowledged as />/ie-
nomena; the mechanism by which they are produced is, however, now
known to be nothing inherent in the light, no essential property re-
curring, but the simple periodicity of conspiring or counteracting
wave-action. — Translator.



INTERFERENCE OF LIGHT. 303

Young had not this degree of prudence. He showed
at once that his theory would agree with the phenomena,
but without going beyond mere possibiHty. When at a
later period he arrived at real proofs of it, the public
had other prepossessions, which he was not able to over-
come. However, the experiment, whence our colleague
deduced so memorable a discovery, could not excite the
shadow of a doubt.*

* In the retrospective glance which the author thus gives over the
progress of discovery previous to the period at which Dr. Young first
entered on the field, what we have chiefly to observe is, that up to
that date nothing like a connected view of the physical character of
this wonderful agent had been attained ; a few isolated speculations
had indeed been put forth respecting a theory of emitted molecules
on the one hand, and of waves in an ethereal medium on tlie other;
and a few experimental facts bearing on the choice between such hy-
potheses had been ascertained.

The several distinct phenomena of common reflexion and refrac-
tion, of double refraction, of inflexion or diffraction, and of the
cqloured rings, did not seem to be connected by any common princi-
ple; nor, even, separately considered, could it be said that they were
very satisfactorily explained. It was now the peculiar distinction of
Young to perceive, and to establish in the most incontestable manner,
a great principle of the simplest kind, which at once rendered the
wave hypothesis applicable to the two last-named classes of facts, and
thus directly connected them with the former.

It is not always that we are enabled to trace the first rise and pro-
gress of the idea of a great discovery in the inventor's mind. We can-
not forbear from here noticing, that Dr. Young has left on record the
progress of the first suggestions which occurred to him on the subject
of interference. The first view which presented itself was that of the
analogies furnished by sound, which, as is well known, is conveyed by
means of waves propagated in air. And in the case of two sounds
differing a very little from the same pitch, produced at the same time,
we have, not a continuous sound, but beats, that is, alternations of
sound and silence; the waves in the one case conspiring with and
reinforcing each other, in the other counteracting, neutralizing, and
destroying each other.

But in more special reference to light. Dr. Young's account of the
origin of his ideas is so clear and striking that we must give it in his



304 THOMAS YOUNG.

Two rays, proceeding from the same source by
slightly unequal routes, crossed one another at a cer-

own words: " It was in May, 1801, that I discovered by reflecting on
the beautiful experiments of Newton, a law which appears to me to
account for a greater variety of interesting phenomena than any other
optical principle that has yet been made known. I shall endeavour
to explain this law by a comparison: Suppose a number of equal
waves of water to move upon the surface of a stagnant lake, with a
certain constant velocity, and to enter a narrow channel leading out
of the lake; — suppose, then, another similar cause to have excited
another equal series of waves, which arrive at the same channel with
the same velocity, and at the same time with the first. Neither series
of waves will destroy the other, but their effects will be combined; if
they enter the channel in such a manner that the elevations of the one
series coincide with those of the other, they must together produce a



Online LibraryF. (François) AragoBiographies of distinguished scientific men (Volume 2) → online text (page 23 of 38)