which are worthy of attention: Here, in the first instance, we have a
rich spectrum produced by the decomposition of the beam (from L, fig.
9). One face of the prism (P) is protected by a diaphragm (not shown
in the figure), with a longitudinal slit, through which the beam
passes into the prism. It emerges decomposed at the other side. I
permit the colours to pass through a cylindrical lens (C), which so
squeezes them together as to produce upon the screen a sharply defined
rectangular image of the longitudinal slit. In that image the colours
are reblended, and it is perfectly white. Between the prism and the
cylindrical lens may be seen the colours, tracking themselves through
the dust of the room. Cutting off the more refrangible fringe by a
card, the rectangle is seen red: cutting off the less refrangible
fringe, the rectangle is seen blue. By means of a thin glass prism
(W), I deflect one portion of the colours, and leave the residual
portion. On the screen are now two coloured rectangles produced in
this way. These are _complementary_ colours--colours which, by their
union, produce white. Note, that by judicious management, one of these
colours is rendered _yellow_, and the other _blue_. I withdraw the
thin prism; yellow and blue immediately commingle, and we have _white_
as the result of their union. On our way, then, we remove the fallacy,
first exposed by Wünsch, and afterwards independently by Helmholtz,
that the mixture of blue and yellow lights produces green.
Restoring the circular aperture, we obtain once more a spectrum like
that of Newton. By means of a lens, we can gather up these colours,
and build them together, not to an image of the aperture, but to an
image of the carbon-points themselves.
Finally, by means of a rotating disk, on which are spread in sectors
the colours of the spectrum, we blend together the prismatic colours
in the eye itself, and thus produce the impression of whiteness.
Having unravelled the interwoven constituents of white light, we have
next to inquire, What part the constitution so revealed enables this
agent to play in Nature? To it we owe all the phenomena of colour, and
yet not to it alone; for there must be a certain relationship between
the ultimate particles of natural bodies and white light, to enable
them to extract from it the luxury of colour. But the function of
natural bodies is here _selective_, not _creative_. There is no colour
_generated_ by any natural body whatever. Natural bodies have showered
upon them, in the white light of the sun, the sum total of all
possible colours; and their action is limited to the sifting of that
total--the appropriating or absorbing of some of its constituents,
and the rejecting of others. It will fix this subject in your minds if
I say, that it is the portion of light which they reject, and not that
which they appropriate or absorb, that gives bodies their colours.
Let us begin our experimental inquiries here by asking, What is the
meaning of blackness? Pass a black ribbon through the colours of the
spectrum; it quenches all of them. The meaning of blackness is thus
revealed--it is the result of the absorption of all the constituents
of solar light. Pass a red ribbon through the spectrum. In the red
light the ribbon is a vivid red. Why? Because the light that enters
the ribbon is not quenched or absorbed, but in great part sent back to
the eye. Place the same ribbon in the green of the spectrum; it is
black as jet. It absorbs the green light, and renders the space on
which that light falls a space of intense darkness. Place a green
ribbon in the green of the spectrum. It shines vividly with its proper
colour; transfer it to the red, it is black as jet. Here it absorbs
all the light that falls upon it, and offers mere darkness to the eye.
Thus, when white light is employed, the red sifts it by quenching the
green, and the green sifts it by quenching the red, both exhibiting
the residual colour. The process through which natural bodies acquire
their colours is therefore a _negative_ one. The colours are produced
by subtraction, not by addition. This red glass is red because it
destroys all the more refrangible rays of the spectrum. This blue
liquid is blue because it destroys all the less refrangible rays. Both
together are opaque because the light transmitted by the one is
quenched by the other. In this way, by the union of two transparent
substances, we obtain a combination as dark as pitch to solar light.
This other liquid, finally, is purple because it destroys the green
and the yellow, and allows the terminal colours of the spectrum to
pass unimpeded. From the blending of the blue and the red this
gorgeous purple is produced.
One step further for the sake of exactness. The light which falls upon
a body is divided into two portions, one of which is reflected from
the surface of the body; and this is of the same colour as the
incident light. If the incident light be white, the superficially
reflected light will also be white. Solar light, for example,
reflected from the surface of even a black body, is white. The
blackest camphine smoke in a dark room, through which a sunbeam passes
from an aperture in the window-shutter, renders the track of the beam
white, by the light scattered from the surfaces of the soot particles.
The moon appears to us as if
'Clothed in white samite, mystic, wonderful;'
but were it covered with the blackest velvet it would still hang as a
white orb in the heavens, shining upon our world substantially as it
does now.
§ 8. _Colours of Pigments as distinguished from Colours of Light_.
The second portion of the incident light enters the body, and upon its
treatment there the colour of the body depends. And here a moment may
properly be given to the analysis of the action of pigments upon
light. They are composed of fine particles mixed with a vehicle; but
how intimately soever the particles may be blended, they still remain
particles, separated, it may be, by exceedingly minute distances, but
still separated. To use the scientific phrase, they are not optically
continuous. Now, wherever optical continuity is ruptured we have
reflection of the incident light. It is the multitude of reflections
at the limiting surfaces of the particles that prevents light from
passing through snow, powdered glass, or common salt. The light here
is exhausted in echoes, not extinguished by true absorption. It is the
same kind of reflection that renders the thunder-cloud so impervious
to light. Such a cloud is composed of particles of water, mixed with
particles of air, both separately transparent, but practically opaque
when thus mixed together.
In the case of pigments, then, the light is _reflected_ at the
limiting surfaces of the particles, but it is in part _absorbed_
within the particles. The reflection is necessary to send the light
back to the eye; the absorption is necessary to give the body its
colour. The same remarks apply to flowers. The rose is red, in virtue,
not of the light reflected from its surface, but of light which has
entered its substance, which has been reflected from surfaces within,
and which, in returning _through_ the substance, has had its green
extinguished. A similar process in the case of hard green leaves
extinguishes the red, and sends green light from the body of the
leaves to the eye.
All bodies, even the most transparent, are more or less absorbent of
light. Take the case of water. A glass cell of clear water interposed
in the track of our beam does not perceptibly change any one of the
colours of the spectrum. Still absorption, though insensible, has
here occurred, and to render it sensible we have only to increase the
depth of the water through which the light passes. Instead of a cell
an inch thick, let us take a layer, ten or fifteen feet thick: the
colour of the water is then very evident. By augmenting the thickness
we absorb more of the light, and by making the thickness very great we
absorb the light altogether. Lampblack or pitch can do no more, and
the only difference in this respect between them and water is that a
very small depth in their case suffices to extinguish all the light.
The difference between the highest known transparency and the highest
known opacity is one of degree merely.
If, then, we render water sufficiently deep to quench all the light;
and if from the interior of the water no light reaches the eye, we
have the condition necessary to produce blackness. Looked properly
down upon, there are portions of the Atlantic Ocean to which one would
hardly ascribe a trace of colour: at the most a tint of dark indigo
reaches the eye. The water, in fact, is practically _black_, and this
is an indication both of its depth and purity. But the case is
entirely changed when the ocean contains solid particles in a state of
mechanical suspension, capable of sending the light impinging on them
back to the eye.
Throw, for example, a white pebble, or a white dinner plate, into the
blackest Atlantic water; as it sinks it becomes greener and greener,
and, before it disappears, it reaches a vivid blue green. Break such a
pebble, or plate, into fragments, these will behave like the unbroken
mass: grind the pebble to powder, every particle will yield its
modicum of green; and if the particles be so fine as to remain
suspended in the water, the scattered light will be a uniform green.
Hence the greenness of shoal water. You go to bed with the black water
of the Atlantic around you. You rise in the morning, find it a vivid
green, and correctly infer that you are crossing the Bank of
Newfoundland. Such water is found charged with fine matter in a state
of mechanical suspension. The light from the bottom may sometimes come
into play, but it is not necessary. The subaqueous foam, generated by
the screw or paddle-wheels of a steamer, also sends forth a vivid
green. The foam here furnishes a _reflecting surface_, the water
between the eye and it the _absorbing medium_.
Nothing can be more superb than the green of the Atlantic waves when
the circumstances are favourable to the exhibition of the colour. As
long as a wave remains unbroken no colour appears, but when the foam
just doubles over the crest like an Alpine snow-cornice, under the
cornice we often see a display of the most exquisite green. It is
metallic in its brilliancy. The foam is first illuminated, and it
scatters the light in all directions; the light which passes through
the higher portion of the wave alone reaches the eye, and gives to
that portion its matchless colour. The folding of the wave, producing,
as it does, a series of longitudinal protuberances and furrows which
act like cylindrical lenses, introduces variations in the intensity of
the light, and materially enhances its beauty.
We are now prepared for the further consideration of a point already
adverted to, and regarding which error long found currency. You will
find it stated in many books that blue light and yellow light mixed
together, produce green. But blue and yellow have been just proved to
be complementary colours, producing white by their mixture. The
mixture of blue and yellow _pigments_ undoubtedly produces green, but
the mixture of pigments is a totally different thing from the mixture
of lights.
Helmholtz has revealed the cause of the green produced by a mixture of
blue and yellow pigments. No natural colour is _pure_. A blue liquid,
or a blue powder, permits not only the blue to pass through it, but a
portion of the adjacent green. A yellow powder is transparent not only
to the yellow light, but also in part to the adjacent green. Now, when
blue and yellow are mixed together, the blue cuts off the yellow, the
orange, and the red; the yellow, on the other hand, cuts off the
violet, the indigo, and the blue. Green is the only colour to which
both are transparent, and the consequence is that, when white light
falls upon a mixture of yellow and blue powders, the green alone is
sent back to the eye. You have already seen that the fine blue
ammonia-sulphate of copper transmits a large portion of green, while
cutting off all the less refrangible light. A yellow solution of
picric acid also allows the green to pass, but quenches all the more
refrangible light. What must occur when we send a beam through both
liquids? The experimental answer to this question is now before you:
the green band of the spectrum alone remains upon the screen.
The impurity of natural colours is strikingly illustrated by an
observation recently communicated to me by Mr. Woodbury. On looking
through a blue glass at green leaves in sunshine, he saw the
superficially reflected light blue. The light, on the contrary, which
came from the body of the leaves was crimson. On examination, I found
that the glass employed in this observation transmitted both ends of
the spectrum, the red as well as the blue, and that it quenched the
middle. This furnished an easy explanation of the effect. In the
delicate spring foliage the blue of the solar light is for the most
part absorbed, and a light, mainly yellowish green, but containing a
considerable quantity of red, escapes from the leaf to the eye. On
looking at such foliage through the violet glass, the green and the
yellow are stopped, and the red alone reaches the eye. Thus regarded,
therefore, the leaves appear like faintly blushing roses, and present
a very beautiful appearance. With the blue ammonia-sulphate of copper,
which transmits no red, this effect is not obtained.
As the year advances the crimson gradually hardens to a coppery red;
and in the dark green leaves of old ivy it is almost absent.
Permitting a beam of white light to fall upon fresh leaves in a dark
room, the sudden change from green to red, and from red back to green,
when the violet glass is alternately introduced and withdrawn, is very
surprising. Looked at through the same glass, the meadows in May
appear of a warm purple. With a solution of permanganate of potash,
which, while it quenches the centre of the spectrum, permits its ends
to pass more freely than the violet glass, excellent effects are also
obtained.[7]
This question of absorption, considered with reference to its
molecular mechanism, is one of the most subtle and difficult in
physics. We are not yet in a condition to grapple with it, but we
shall be by-and-by. Meanwhile we may profitably glance back on the web
of relations which these experiments reveal to us. We have, firstly,
in solar light an agent of exceeding complexity, composed of
innumerable constituents, refrangible in different degrees. We find,
secondly, the atoms and molecules of bodies gifted with the power of
sifting solar light in the most various ways, and producing by this
sifting the colours observed in nature and art. To do this they must
possess a molecular structure commensurate in complexity with that of
light itself. Thirdly, we have the human eye and brain, so organized
as to be able to take in and distinguish the multitude of impressions
thus generated. The light, therefore, at starting is complex; to sift
and select it as they do, natural bodies must be complex; while to
take in the impressions thus generated, the human eye and brain,
however we may simplify our conceptions of their action,[8] must be
highly complex.
Whence this triple complexity? If what are called material purposes
were the only end to be served, a much simpler mechanism would be
sufficient. But, instead of simplicity, we have prodigality of
relation and adaptation--and this, apparently, for the sole purpose of
enabling us to see things robed in the splendours of colour. Would it
not seem that Nature harboured the intention of educating us for other
enjoyments than those derivable from meat and drink? At all events,
whatever Nature meant--and it would be mere presumption to dogmatize
as to what she meant--we find ourselves here, as the upshot of her
operations, endowed, not only with capacities to enjoy the materially
useful, but endowed with others of indefinite scope and application,
which deal alone with the beautiful and the true.
LECTURE II.
ORIGIN OF PHYSICAL THEORIES
SCOPE OF THE IMAGINATION
NEWTON AND THE EMISSION THEORY
VERIFICATION OF PHYSICAL THEORIES
THE LUMINIFEROUS ETHER
WAVE THEORY OF LIGHT
THOMAS YOUNG
FRESNEL AND ARAGO
CONCEPTION OF WAVE-MOTION
INTERFERENCE OF WAVES
CONSTITUTION OF SOUND-WAVES
ANALOGIES OF SOUND AND LIGHT
ILLUSTRATIONS OF WAVE-MOTION
INTERFERENCE OF SOUND-WAVES
OPTICAL ILLUSTRATIONS
PITCH AND COLOUR
LENGTHS OF THE WAVES OF LIGHT AND RATES OF VIBRATION OF
THE ETHER-PARTICLES
INTERFERENCE OF LIGHT
PHENOMENA WHICH FIRST SUGGESTED THE UNDULATORY THEORY
BOYLE AND HOOKE
THE COLOURS OF THIN PLATES
THE SOAP-BUBBLE
NEWTON'S RINGS
THEORY OF 'FITS'
ITS EXPLANATION OF THE RINGS
OVER-THROW OF THE THEORY
DIFFRACTION OF LIGHT
COLOURS PRODUCED BY DIFFRACTION
COLOURS OF MOTHER-OF-PEARL.
§ 1. _Origin and Scope of Physical Theories_.
We might vary and extend our experiments on Light indefinitely, and
they certainly would prove us to possess a wonderful mastery over the
phenomena. But the vesture of the agent only would thus be revealed,
not the agent itself. The human mind, however, is so constituted that
it can never rest satisfied with this outward view of natural things.
Brightness and freshness take possession of the mind when it is
crossed by the light of principles, showing the facts of Nature to be
organically connected.
Let us, then, inquire what this thing is that we have been generating,
reflecting, refracting and analyzing.
In doing this, we shall learn that the life of the experimental
philosopher is twofold. He lives, in his vocation, a life of the
senses, using his hands, eyes, and ears in his experiments: but such a
question as that now before us carries him beyond the margin of the
senses. He cannot consider, much less answer, the question, 'What is
light?' without transporting himself to a world which underlies the
sensible one, and out of which all optical phenomena spring. To
realise this subsensible world the mind must possess a certain
pictorial power. It must be able to form definite images of the things
which that world contains; and to say that, if such or such a state of
things exist in the subsensible world, then the phenomena of the
sensible one must, of necessity, grow out of this state of things.
Physical theories are thus formed, the truth of which is inferred from
their power to explain the known and to predict the unknown.
This conception of physical theory implies, as you perceive, the
exercise of the imagination--a word which seems to render many
respectable people, both in the ranks of science and out of them,
uncomfortable. That men in the ranks of science should feel thus is, I
think, a proof that they have suffered themselves to be misled by the
popular definition of a great faculty, instead of observing its
operation in their own minds. Without imagination we cannot take a
step beyond the bourne of the mere animal world, perhaps not even to
the edge of this one. But, in speaking thus of imagination, I do not
mean a riotous power which deals capriciously with facts, but a
well-ordered and disciplined power, whose sole function is to form
such conceptions as the intellect imperatively demands. Imagination,
thus exercised, never really severs itself from the world of fact.
This is the storehouse from which its materials are derived; and the
magic of its art consists, not in creating things anew, but in so
changing the magnitude, position, grouping, and other relations of
sensible things, as to render them fit for the requirements of the
intellect in the subsensible world.[9]
Descartes imagined space to be filled with something that transmitted
light _instantaneously_. Firstly, because, in his experience, no
measurable interval was known to exist between the appearance of a
flash of light, however distant, and its effect upon consciousness;
and secondly, because, as far as his experience went, no physical
power is conveyed from place to place without a vehicle. But his
imagination helped itself farther by illustrations drawn from the
world of fact. 'When,' he says,' one walks in darkness with staff in
hand, the moment the distant end of the staff strikes an obstacle the
hand feels it. This explains what might otherwise be thought strange,
that the light reaches us instantaneously from the sun. I wish thee to
believe that light in the bodies that we call luminous is nothing more
than a very brisk and violent motion, which, by means of the air and
other transparent media, is conveyed to the eye, exactly as the shock
through the walking-stick reaches the hand of a blind man. This is
instantaneous, and would be so even if the intervening distance were
greater than that between earth and heaven. It is therefore no more
necessary that anything material should reach the eye from the
luminous object, than that something should be sent from the ground to
the hand of the blind man when he is conscious of the shock of his
staff.' The celebrated Robert Hooke at first threw doubt upon this
notion of Descartes, but he afterwards substantially espoused it. The
belief in instantaneous transmission was destroyed by the discovery of
Roemer referred to in our last lecture.
§ 2. _The Emission Theory of Light_.
The case of Newton still more forcibly illustrates the position, that
in forming physical theories we draw for our materials upon the world
of fact. Before he began to deal with light, he was intimately
acquainted with the laws of elastic collision, which all of you have
seen more or less perfectly illustrated on a billiard-table. As
regards the collision of sensible elastic masses, Newton knew the
angle of incidence to be equal to the angle of reflection, and he also
knew that experiment, as shown in our last lecture (fig. 3), had
established the same law with regard to light. He thus found in his
previous knowledge the material for theoretic images. He had only to
change the magnitude of conceptions already in his mind to arrive at
the Emission Theory of Light. Newton supposed light to consist of
elastic particles of inconceivable minuteness, shot out with
inconceivable rapidity by luminous bodies. Optical reflection
certainly occurred _as if_ light consisted of such particles, and this
was Newton's justification for introducing them.
But this is not all. In another important particular, also, Newton's
conceptions regarding the nature of light were influenced by his
previous knowledge. He had been pondering over the phenomena of
gravitation, and had made himself at home amid the operations of this
universal power. Perhaps his mind at this time was too freshly and too
deeply imbued with these notions to permit of his forming an
unfettered judgment regarding the nature of light. Be that as it may,
Newton saw in Refraction the result of an attractive force exerted on
the light-particles. He carried his conception out with the most
severe consistency. Dropping vertically downwards towards the earth's
surface, the motion of a body is accelerated as it approaches the
earth. Dropping downwards towards a horizontal surface--say from air
on to glass or water--the velocity of the light-particles, when they
came close to the surface, is, according to Newton, also accelerated.
Approaching such a surface obliquely, he supposed the particles, when
close to it, to be drawn down upon it, as a projectile is deflected by
gravity to the surface of the earth. This deflection was, according to
Newton, the refraction seen in our last lecture (fig. 4). Finally, it
was supposed that differences of colour might be due to differences
in the 'bigness' of the particles. This was the physical theory of
light enunciated and defended by Newton; and you will observe that it
simply consists in the transference of conceptions, born in the world
of the senses, to a subsensible world.