_Newton’s Theory of the Colours of Natural Bodies explained—Objections
to it stated—New Classification of Colours—Outline of a New Theory
proposed._
If the objects of the material world had been illuminated with white
light, all the particles of which possessed the same degree of
refrangibility, and were equally acted upon by the bodies on which
they fall, all nature would have shone with a leaden hue, and all the
combinations of external objects, and all the features of the human
countenance, would have exhibited no other variety but that which
they possess in a pencil sketch or a China-ink drawing. The rainbow
itself would have dwindled into a narrow arch of white light,—the
stars would have shone through a gray sky,—and the mantle of a wintry
twilight would have replaced the golden vesture of the rising and the
setting sun. But He who has exhibited such matchless skill in the
organization of material bodies, and such exquisite taste in the forms
upon which they are modelled, has superadded that ethereal beauty which
enhances their more permanent qualities, and presents them to us in
the ever-varying colours of the spectrum. Without this the foliage of
vegetable life might have filled the eye and fostered the fruit which
it veils,—but the youthful green of its spring would have been blended
with the dying yellow of its autumn. Without this the diamond might
have displayed to science the beauty of its forms, and yielded to the
arts its adamantine virtues;—but it would have ceased to shine in the
chaplet of beauty, and to sparkle in the diadem of princes. Without
this the human countenance might have expressed all the sympathies of
the heart, but the “purple light of love” would not have risen on the
cheek, nor the hectic flush been the herald of its decay.
The gay colouring with which the Almighty has decked the pale marble
of nature is not the result of any quality inherent in the coloured
body, or in the particles by which it may be tinged, but is merely a
property of the light in which they happen to be placed. Newton was the
first person who placed this great truth in the clearest evidence. He
found that all bodies, whatever were their peculiar colours, exhibited
these colours only in white light. When they were illuminated by
homogeneous _red_ light they appeared _red_, by homogeneous _yellow_
light, _yellow_, and so on, “their colours being most brisk and vivid
under the influence of their own daylight colours.” The leaf of a
plant, for example, appeared _green_ in the white light of day, because
it had the property of reflecting that light in greater abundance than
any other. When it was placed in homogeneous _red_ light, it could no
longer appear _green_, because there was no green light to reflect; but
it reflected a portion of red light, because there was some red in the
compound green which it had the property of reflecting. Had the leaf
originally reflected a pure homogeneous green, unmixed with red, and
reflected no white light from its outer surface, it would have appeared
quite black in pure homogeneous red light, as this light does not
contain a single ray which the leaf was capable of reflecting. Hence
the colours of material bodies are owing to the property which they
possess of stopping certain rays of white light, while they reflect
or transmit to the eye the rest of the rays of which white light is
composed.
So far the Newtonian doctrine of colours is capable of rigid
demonstration; but its author was not content with carrying it thus
far: he sought to determine the manner in which particular rays are
stopped, while others are reflected or transmitted; and the result of
this profound inquiry was his theory of the colours of natural bodies,
which was communicated to the Royal Society on the 10th February,
1675. This theory is perhaps the loftiest of all his speculations; and
though, as a physical generalization, it stands on a perishable basis,
and must soon be swept away in the progress of science, it yet bears
the deepest impress of the grasp of his powerful intellect.
The principles upon which this theory is founded are the following:—
1. Bodies that have the greatest refractive powers reflect the greatest
quantity of light; and at the confines of equally refracting media
there is no reflection.
2. The least particles of almost all natural bodies are in some measure
transparent.
3. Between the particles of bodies are many pores or spaces, either
empty or filled with media of less density than the particles.
4. The particles of bodies and their pores, or the spaces between the
particles, have some definite size.
Upon these principles Newton explains the origin of _transparency_,
_opacity_, and _colour_.
_Transparency_ he considers as arising from the particles and their
intervals or pores being too small to cause reflection at their common
surfaces,[19] so that all the light which enters transparent bodies
passes through them without any portion of it being turned from its
path by reflection. If we could obtain, for example, a film of mica
whose thickness does not exceed two-thirds of the millionth part of an
inch, all the light which fell upon it would pass through it, and none
would be reflected. If this film was then cut into fragments, a number
of such fragments would constitute a bundle, which would also transmit
all the light which fell upon it, and be perfectly transparent.
_Opacity_ in bodies arises, he thinks, from an opposite cause, viz.
when the parts of bodies are of such a size as to be capable of
reflecting the light which falls upon them, in which case the light is
“stopped or stifled” by the multitude of reflections.
The _colours_ of natural bodies have, in the Newtonian hypothesis, the
same origin as the colours of thin plates, their transparent particles,
according to their several sizes, reflecting rays of one colour,
and transmitting those of another. “For if a thinned or plated body
which, being of an uneven thickness, appears all over of one uniform
colour, should be slit into threads, or broken into fragments of the
same thickness with the plate or film, every thread or fragment should
keep its colour, and consequently, a heap of such threads or fragments
should constitute a mass or powder of the same colour which the plate
exhibited before it was broken: and the parts of all natural bodies
being like so many fragments of a plate, must, on the same grounds,
exhibit the same colour.”
Such is the theory of the colours of natural bodies, stated as
clearly and briefly as we can. It has been very generally admitted
by philosophers, both of our own and of other countries, and has
been recently illustrated and defended by a French philosopher of
distinguished eminence. That this theory affords the true explanation
of certain colours, or, to speak more correctly, that certain colours
in natural bodies are the colours of thin plates, cannot be doubted;
but it will not be difficult to show that it is quite inapplicable to
that great class of phenomena which may be considered as representing
the colours of natural bodies.
The first objection to the Newtonian theory is the total absence of
all reflected light from the particles of transparent coloured media,
such as coloured gems, coloured glasses, and coloured fluids. This
objection was urged long ago by Mr. Delaval, who placed coloured fluids
on black grounds, and never could perceive the least trace of the
reflected tints. I have repeated the experiment with every precaution,
and with every variation that I could think of, and I consider it as
an established fact, that in such coloured bodies the complementary
reflected colour cannot be rendered visible. If the fluid, for example,
be _red_, the _green_ light from which the red has been separated
ought to appear either directly by looking into the coloured mass, or
ought to be recognised by its influence in modifying the light really
reflected; but as it cannot be seen, we must conclude that it has not
been reflected, but has been destroyed by some other property of the
coloured body.
A similar objection may be drawn from the disappearance of the
transmitted complementary colour in the leaves of plants and petals
of flowers. I have ascertained from numerous experiments, that the
transmitted colour is almost invariably the same with the reflected
colour, and that the same holds true with the coloured juices expressed
from them. The complementary tints are never seen, and wherever
there has been any thing like an approximation to two tints, I have
invariably found that it arose from there being two different coloured
juices existing in different sides of the leaf.
In the phenomena of the light transmitted by coloured glasses, there
are some peculiarities which, we think, demonstrate that their colours
are not those of thin plates. The light, for example, transmitted
through a particular kind of blue glass, has a blue colour of such a
peculiar composition that there is no blue in any of the orders of
colours in thin plates which has any resemblance to it. It is entirely
destitute of the red rays which form the middle of the red space in
the spectrum; so that the particles on which the colour depends must
reflect the middle red rays, and transmit those on each side of it,—a
property which cannot be deduced from the Newtonian doctrine.
The explanation of _opacity_, as arising from a multitude of
reflections, is liable to the same objection which we have urged
against the explanation of colour. In order to appreciate its weight,
we must distinguish opacity into two kinds, namely, the _opacity of
whiteness_ and the _opacity of blackness_. Those bodies which possess
the power of reflection in the highest degree, such as white metals,
chalk, and plaster of Paris, never reflect more than one-half of the
light which falls upon them. The other half of the incident light is,
according to Newton, lost by a multitude of reflections. But how is
it lost? Reflection merely changes the direction of the particles of
light, so that they must again emerge from the body, unless they are
reflected into fixed returning orbits, which detain them for ever in a
state of motion within the body. In the case of black opacity, such as
that of coal, which reflects from its first surface only 1/25th of the
white light, the difficulty is still greater, and we cannot conceive
how any system of interior reflections could so completely stifle
24/25ths of the whole incident light, without some of it returning to
the eye in a visible form.
In determining the constitution of bodies that produce _transparency_
and _blackness_, the Newtonian theory encounters a difficulty which
its author has by no means surmounted. Transparency, as we have
already seen, arises from the “particles and their interstices being
too small to cause reflections in their common surfaces,” that is,
they must be “less than any of those which exhibit colours,” or
“less than is requisite to reflect the _white_ and very faint blue
of the first order. But _this is the very same constitution which
produces blackness_ by reflection, and in order to explain the cause
of blackness by transmission, or black opacity, Newton is obliged to
introduce a new principle.
“For the production of _black_,” says he, “the corpuscles must be less
than any of those which exhibit colours. For at all greater sizes there
is too much light reflected to constitute this colour. But if they
be supposed a little less than is requisite to reflect the white and
very faint blue of the first order, they will reflect so very little
light as to appear intensely black, _and yet may perhaps variously
refract[20] it_ to and fro within themselves so long, until it happens
to be stifled and lost, by which means they will appear black in all
positions of the eye, without any transparency.”
This very remarkable passage exhibits, in a striking manner, the
perplexity in which our author was involved by the difficulties of his
subject. As the particles which produce blackness by reflection are
necessarily so small as to exclude the existence of any reflective
forces, he cannot ascribe the loss of the intromitted light, as he does
in the case of white opacity, to “a multitude of reflections;” and
therefore he is compelled to have recourse to _refracting forces_ to
perform the same office. The reluctance with which he avails himself
of this expedient is well marked in the mode of expression which he
adopts; and I am persuaded that when he wrote the above passage, he
felt the full force of the objections to this hypothesis, which cannot
fail to present themselves. As the size of the particles which produce
blackness are intermediate between those which produce transparency
and those which produce colour, approaching closely to the latter, it
is difficult to conceive why _they_ should refract the intromitted
light, while the greater and smaller particles, and even those almost
of the same size, should be destitute of that property. It is, besides,
not easy to understand how a refraction can take place within bodies
which shall stifle all the light, and prevent it from emerging. Nay, we
may admit the existence of such refractions, and yet understand how, by
a compensation in their direction, the refracted rays may all emerge
from the opaque body.
The force of these objections is tacitly recognised in Pemberton’s View
of Sir Isaac Newton’s Philosophy;[21] and as Newton not only read and
approved of that work, but even perused a great part of it along with
its author, we may fairly consider the opinion there stated to be his
own.
“For producing _black_, the particles ought to be smaller than for
exhibiting any of the colours, viz. of a size answering to the
thickness of the bubble, whereby reflecting little or no light, it
appears colourless; _but yet they must not be too small_, for that will
make them transparent _through deficiency of reflections_ in the inward
parts of the body, sufficient to stop the light from going through it;
but they must be of a size _bordering upon that_ disposed to reflect
the faint blue of the first order, which affords an evident reason why
blacks usually partake a little of that colour.” In this passage all
idea of refraction is abandoned, and that precise degree of size is
assumed for the particles which leaves a small power of reflection,
which is deemed sufficient to prevent the body from becoming
transparent; that is, sufficient to render it opaque or black.
The last objection which we shall state to this theory is one to which
we attach great weight, and, as it is founded on discoveries and views
which have been published since the time of Newton, we venture to
believe, that, had he been aware of them, he would never have proposed
the theory which we are considering.
When light falls upon a thin film such as AEC, _fig. 9_, p. 80, so
as to produce the colours of thin plates, it follows, from Sir Isaac
Newton’s theory of fits, that a portion of the light is, as usual,
reflected at the first surface AE,[22] while the light which forms the
coloured image is that which is reflected from the second surface EC,
so that all the colours of thin plates are diluted with the white light
reflected from the first surface. Now, in the modern theory, which
ascribes the colours of thin plates to the interference of the light
reflected from the second surface EC, with the light reflected from the
first surface AE, the resulting tint arises from the combination of
these two pencils, and consequently there is no white light reflected
from the surface AE. In like manner, when the thickness of the film is
such that the two interfering pencils completely destroy one another,
and produce black, there is not a ray of light reflected from the first
surface. Here, then, we have a criterion for deciding between the
theory of fits and the theory of interference; for if there is no white
light reflected from the first surface AE, the theory of fits must be
rejected. In a remarkable phenomenon of blackness arising from minute
fibres, which I have had occasion to describe, there was no perceptible
reflection from the surface of the fibres;[23] and M. Fresnel describes
an experiment made to determine the same point, and states the result
of it to have been unequivocally in favour of the doctrine of
interference.
In order to apply this important fact, let us take a piece of coal, one
of the blackest and most opaque of all substances, and which does not
reflect to the eye a single ray out of those which enter its substance.
The size of its particles is so small, that they are incapable of
reflecting light. When a number of these particles are placed together,
so as to form a surface, and other particles behind them, so as to form
a solid, they will not acquire by this process the power of reflection;
and consequently, a piece of coal so composed should be destitute of
the property of reflecting light from its first surface. But this is
not the case,—light is abundantly reflected from the first surface of
the coal, and consequently, its elementary particles must possess the
same power. Hence the blackness of coal must be ascribed to some other
cause than to the minuteness of its transparent atoms.
To transparent bodies this argument has a similar application. As their
atoms are still less than those of black bodies, their inability to
reflect light is still greater, and hence arises their transparency.
But the particles forming the surface of such bodies do reflect light,
and, therefore, their transparency must have another origin.
In the case of coloured bodies, too, the particles forming their
surfaces reflect white light like those of all other bodies, so that
these particles cannot produce colour on the same principles as those
of thin plates. In many of those cases of colour which seem to depend
upon the minuteness of the particles of the body, the reflection of
white light may nevertheless be observed, but this will be found
to arise from a thin transparent film, behind which the colorific
particles are placed.
Whatever answer may be given to these objections, we think it will
be admitted by those who have studied the subject most profoundly,
that a satisfactory theory of the colours of natural bodies is still a
_desideratum_ in science. How far we may be able to approach to it in
the present state of optics the reader will judge from the following
views.
Colours may be arranged into seven classes, each of which depends upon
different principles.
1. Transparent coloured fluids—transparent coloured gems—transparent
coloured glasses—coloured powders—and the colours of the leaves and
flowers of plants.
2. Oxidations on metals—colours of Labrador feldspar—colours of
precious and hydrophanous opal, and other opalescences—the colours of
the feathers of birds, of the wings of insects, and of the scales of
fishes.
3. Superficial colours, as those of mother-of-pearl and striated
surfaces.
4. Opalescences and colours in composite crystals having double
refraction.
5. Colours from the absorption of common and polarized light by doubly
refracting crystals.
6. Colours at the surfaces of media of different dispersive powers.
7. Colours at the surface of media in which the reflecting forces
extend to different distances, or follow different laws.
The first two of these classes are the most important. The Newtonian
theory appears to be strictly applicable to the phenomena of the
_second_ class; but those of the first class cannot, we conceive, be
referred to the same cause.
* * * * *
The rays of solar light possess several remarkable physical properties:
They heat—they illuminate—they promote chymical combination—they effect
chymical decompositions—they impart magnetism to steel—they alter
the colours of bodies—they communicate to plants and flowers their
peculiar colours, and are in many cases necessary to the development of
their characteristic qualities. It is impossible to admit for a moment
that these varied effects are produced by a mere mechanical action,
or that they arise from the agitation of the particles of bodies by
the vibration of the ether which is considered to be the cause of
light. Whatever be the difficulties which attach to the theory which
supposes light to consist of material particles, we are compelled, by
its properties, to admit that light acts as if it were material, and
that it enters into combinations with bodies, in order to produce the
effects which we have enumerated.
When a beam of light falls upon a body, and the whole or a part of that
which enters its substance totally disappears, we are entitled to say,
that it is detained by some power exercised by the particles of the
body over the particles of light. When this light is said to be lost
by a multitude of reflections or refractions, the statement is not
only hypothetical, but it is an hypothesis incompatible with optical
principles. That the light detained within bodies has been stopped by
the attractive force of the particles seems to be highly probable,
and the mind will not feel any repugnance to admit that the particles
of all bodies, whether solid, fluid, or aëriform, have a specific
affinity for the particles of light. Considering light, therefore, as
material, it is not difficult to comprehend how it should, like other
elementary substances, enter into combination with bodies, and produce
many chymical and physical effects, but particularly the phenomena of
transparency, opacity, and colour.
In _transparent_ colourless bodies, such as water and glass, the
intromitted light experiences a considerable loss, because a certain
number of its particles are attracted and detained by the atoms of the
water or glass, and the light which emerges is colourless, because the
particles exercise a proportional action over all the simple colours
which compose white light.
When the transparent body has any decided _colour_, such as those
enumerated in Class I., then the particles of the body have exercised
a specific attraction over those rays of white light which are
complementary to those which compose the colour of the transmitted
light. If the transparent body, for example, is _red_, then its
particles have detained the green rays which entered into the incident
light, or certain other rays, which with the red are necessary to
compose white light. In compound bodies, like some of the artificial
glasses, the particles will attract and detain rays of light of
different colours, as may be seen by analyzing the transmitted light
with a prism, which will exhibit a spectrum deprived of all the rays
which have been detained. In black bodies the particles exercise a
powerful attraction over light, and detain all the intromitted rays.
When coloured bodies are opaque, so as to exhibit their colours
principally by reflection, the light which is reflected back to the
observer has received its colour from transmission through part of the
thickness of the body, or, what is the same thing, the colour reflected
to the eye is complementary to that which has been detained by the
particles of the body while the light is passing and repassing through
a thickness terminated by the reflecting surfaces; and as only a part
of this light is reflected, as in the case of leaves and flowers, the
transmitted light must have the same colour as the reflected light.
When coloured bodies exhibit two different colours complementary to
each other, the one seen by reflection and the other by transmission,
it is then highly probable that the colours are those of thin plates,
though there are still other optical principles to which they may be
referred. As the particles of bodies, and the medium which unites
them, or, as the different atoms of a compound body may have different
dispersive powers, while they exercise the same refractive force over a
particular part of the spectrum, the rays for which this compensation
takes place will be transmitted, while part of the complementary light
is reflected.[24] Or in cases where the refractive and dispersive
powers are the same, the reflective forces of the particles may vary
according to a different law, so that at the separating surfaces either
white or coloured light may be reflected.[25]
In those cases of colour where the reflected and the transmitted tints
are not complementary, as in _leaf-gold_, where the former is _yellow_
and the latter _green_;—in _leaf-silver_, where they are _white_
and _blue_, and in certain pieces of fir-wood, where the reflected
light is _whitish yellow_, and the transmitted light a _brilliant
homogeneous red_, we may explain the separation of the colours either
by the principles we have already laid down or by the doctrine of thin
plates. On the first principle, the colour of the reflected light,
which is supposed to be the same as that of the transmitted light, will
be modified by the law according to which the particles of the body
attract different rays out of the beam of white light. In pitch, for
example, the blue rays are first absorbed, so that at small thicknesses
the transmitted light is a fine yellow, while, by the action of a
greater thickness, the yellow itself is absorbed, and the transmitted
light is a bright homogeneous red. Now in leaf-gold the transmitted
colour of thinner films than we can obtain may be yellow, and,
consequently, the light reflected from the first strata of interrupting
faces will be yellow, and will determine the predominant tint of the
reflected light. On the Newtonian doctrine, Mr. Herschel has explained
it by saying, “that the transmitted rays have traversed the whole
thickness of the medium, and therefore undergo many more times the
action of its atoms than those reflected, especially those near the
first surface to which the brighter part of the reflected colour is
due.”
The phenomena of the absorption of common and polarized light, which I
have described in another place,[26] throw much light on the subject
of coloured bodies. The relation of the absorbent action to the axes
of double refraction, and, consequently, to the poles of the molecules
of the crystal, shows how the particles of light attracted by the
molecules of the body will vary, both in their nature and number,
according to the direction in which they approach the molecules; and
explains how the colour of a body may be changed, either temporarily
or permanently, by heat, according as it produces a temporary or a
permanent change in the relative position of the molecules. This is not
the place to enlarge on this subject; but we may be permitted to apply
the idea to the curious experiment of Thenard on phosphorus. When this
substance is rendered pure by repeated distillation, it is transparent,
and transmits yellow light; but when it is thrown in a melted state
into cold water, it becomes jet black. When again melted, it resumes
its original colour and transparency. According to the Newtonian
theory, we must suppose that the atoms of the phosphorus have been
diminished in size by sudden cooling,—an effect which it is not easy
to comprehend; but, according to the preceding views, we may suppose
that the atoms of the phosphorus have been forced by sudden cooling
into relative positions quite different from those which they take
when they slowly assume the solid state, and their poles of maximum
attraction, in place of being turned to one another, are turned in
different directions, and then allowed to exercise their full action in
attracting the intromitted light, and detaining it wholly within the
body.[27]
Before concluding this chapter, there is one topic peculiarly
deserving our notice, namely, the change of colour produced in bodies
by continued exposure to light. The general effect of light is to
diminish or dilute the colours of bodies, and in many cases to deprive
them entirely of their colour. Now, it is not easy to understand how
repeated undulations propagated through a body could diminish the
size of its particles, or how the same effect could be produced by a
multitude of reflections from particle to particle. But if light is
attracted by the particles of bodies, and combines with them, it is
easy to conceive that, when the molecules of a body have combined with
a great number of particles of a green colour, for example, their power
of combination with others will be diminished, and, consequently, the
number of particles of any colour absorbed or detained must diminish
with the time that the body has been exposed to light; that is, these
particles must enter into the transmitted and reflected pencils, and
diminish the intensity of their colour. If the body, for example,
absorbs red light, and transmits and reflects green, then if the
quantity of absorbed red light is diminished, it will enter into the
reflected and transmitted pencils, and, forming white light by its
mixture with a portion of the green rays, will actually dilute them in
the same manner as if a portion of white light had been added.[28]