discourse, and of preventing either failure of knowledge or of memory,
from causing any rent in our picture, I here propose to run rapidly
over a bit of ground which is probably familiar to most of you, but
which I am anxious to make familiar to you all. The waves generated
in the aether by the swinging atoms of luminous bodies are of
different lengths and amplitudes. The amplitude is the width of swing
of the individual particles of the waves. In water-waves it is the
vertical height of the crest above the trough, while the length of the
wave is the horizontal distance between two consecutive crests. The
aggregate of waves emitted by the sun may be broadly divided into two
classes: the one class competent, the other incompetent, to excite
vision. But the light-producing waves differ markedly among
themselves in size, form, and force. The length of the largest of
these waves is about twice that of the smallest, but the amplitude of
the largest is probably a hundred times that of the smallest. Now the
force or energy of the wave, which, expressed with reference to
sensation, means the intensity of the light, is proportional to the
square of the amplitude. Hence the amplitude being one-hundredfold,
the energy of the largest light-giving waves would be ten-thousandfold
that of the smallest. This is not improbable. I use these figures not
with a view to numerical accuracy, but to give you definite ideas of
the differences that probably exist among the light-giving waves. And
if we take the whole range of solar radiation into account--its
non-visual as well as its visual waves--I think it probable that the
force, or energy, of the largest wave is more than a million times
that of the smallest.
Turned into their equivalents of sensation, the different light-waves
produce different colours. Red, for example, is produced by the
largest waves, violet by the smallest, while green is produced by a
wave of intermediate length and amplitude. On entering from air into
a more highly refracting substance, such as glass or water, or the
sulphide of carbon, all the waves are retarded, but the smallest ones
most. This furnishes a means of separating the different classes of
waves from each other; in other words, of analysing the light.
Sent through a refracting prism, the waves of the sun are turned aside
in different degrees from their direct course, the red least, the
violet most. They are virtually pulled asunder, and they paint upon a
white screen placed to receive them 'the solar spectrum.' Strictly
speaking, the spectrum embraces an infinity of colours; but the limits
of language, and of our powers of distinction, cause it to be divided
into seven segments: red, orange, yellow, green, blue, indigo, violet.
These are the seven primary or prismatic colours.
Separately, or mixed in various proportions, the solar waves yield all
the colours observed in nature and employed in art. Collectively,
they give us the impression of whiteness. Pure unsifted solar light
is white; and, if all the wave-constituents of such light be reduced
in the same proportion, the light, though diminished in intensity,
will still be white. The whiteness of snow with the sun shining upon
it, is barely tolerable to the eye. The same snow under an overcast
firmament is still white. Such a firmament enfeebles the light by
reflecting it upwards; and when we stand above a cloud-field--on an
Alpine summit, for instance, or on the top of Snowdon--and see, in the
proper direction, the sun shining on the clouds below us, they appear
dazzlingly white. Ordinary clouds, in fact, divide the solar light
impinging on them into two parts--a reflected part and a transmitted
part, in each of which the proportions of wave-motion which produce
the impression of whiteness are sensibly preserved.
It will be understood that the condition of whiteness would fail if
all the waves were diminished _equally_, or by the same absolute
quantity. They must be reduced _proportionately_, instead of equally.
If by the act of reflection the waves of red light are split into
exact halves, then, to preserve the light white, the waves of yellow,
orange, green, and blue, must also be split into exact halves. In
short, the reduction must take place, not by absolutely equal
quantities, but by equal fractional parts. In white light the
preponderance, as regards energy, of the larger over the smaller waves
must always be immense. Were the case otherwise, the visual
correlative, blue, of the smaller waves would have the upper hand in
our sensations.
Not only are the waves of aether reflected by clouds, by solids, and
by liquids, but when they pass from light air to dense, or from dense
air to light, a portion of the wave-motion is always reflected. Now
our atmosphere changes continually in density from top to bottom. It
will help our conceptions if we regard it as made up of a series of
thin concentric layers, or shells of air, each shell being of the same
density throughout, a small and sudden change of density occurring in
passing from shell to shell. Light would be reflected at the limiting
surfaces of all these shells, and their action would be practically
the same as that of the real atmosphere. And now I would ask your
imagination to picture this act of reflection. What must become of
the reflected light? The atmospheric layers turn their convex
surfaces towards the sun; they are so many convex mirrors of feeble
power; and you will immediately perceive that the light regularly
reflected from these surfaces cannot reach the earth at all, but is
dispersed in space. Light thus reflected cannot, therefore, be the
light of the sky.
But, though the sun's light is not reflected in this fashion from the
aerial layers to the earth, there is indubitable evidence to show that
the light of our firmament is scattered light. Proofs of the most
cogent description could be here adduced; but we need only consider
that we receive light at the same time from all parts of the
hemisphere of heaven. The light of the firmament comes to us across
the direction of the solar rays, and even against the direction of the
solar rays; and this lateral and opposing rush of wave-motion can only
be due to the rebound of the waves from the air itself, or from
something suspended in the air. It is also evident that, unlike the
action of clouds, the solar light is not reflected by the sky in the
proportions which produce white. The sky is blue, which indicates an
excess of the shorter waves. In accounting for the colour of the sky,
the first question suggested by analogy would undoubtedly be, Is not
the air blue? The blueness of the air has, in fact, been given as a
solution of the blueness of the sky. But how, if the air be blue, can
the light of sunrise and sunset, which travels through vast distances
of air, be yellow, orange, or even red? The passage of white solar
light through a blue medium could by no possibility redden the light.
The hypothesis of a blue air is therefore untenable. In fact the
agent, whatever it is, which sends us the light of the sky, exercises
in so doing a dichroitic action. The light reflected is blue, the
light transmitted is orange or red. A marked distinction is thus
exhibited between the matter of the sky, and that of an ordinary
cloud, which exercises no such dichroitic action.
By the scientific use of the imagination we may hope to penetrate this
mystery. The cloud takes no note of size on the part of the waves of
aether, but reflects them all alike. It exercises no selective
action. Now the cause of this may be that the cloud particles are so
large, in comparison with the waves of aether, as to reflect them all
indifferently. A broad cliff reflects an Atlantic roller as easily as
a ripple produced by a seabird's wing; and in the presence of large
reflecting surfaces, the existing differences of magnitude among the
waves of aether may disappear. But supposing the reflecting
particles, instead of being very large, to be very small in comparison
with the size of the waves. In this case, instead of the whole wave
being fronted and thrown back, a small portion only is shivered off.
The great mass of the wave passes over such a particle without
reflection. Scatter, then, a handful of such minute foreign particles
in our atmosphere, and set imagination to watch their action upon the
solar waves. Waves of all sizes impinge upon the particles, and you
see at every collision a portion of the impinging wave struck off; all
the waves of the spectrum, from the extreme red to the extreme violet,
being thus acted upon.
Remembering that the red waves stand to the blue much in the relation
of billows to ripples, we have to consider whether those extremely
small particles are competent to scatter all the waves in the same
proportion. If they be not--and a little reflection will make it
clear that they are not--the production of colour must be an incident
of the scattering. Largeness is a thing of relation; and the smaller
the wave, the greater is the relative size of any particle on which
the wave impinges, and the greater also the ratio of the portion
scattered to the total wave A pebble, placed in the way of the
ring-ripples produced by heavy raindrops on a tranquil pond, will
scatter a large fraction of each ripple, while the fractional part of
a larger wave thrown back by the same pebble might be infinitesimal.
Now we have already made it clear to our minds that to preserve the
solar light white, its constituent proportions must not be altered;
but in the act of division performed by these very small particles the
proportions are altered; an undue fraction of the smaller waves is
scattered by the particles, and, as a consequence, in the scattered
light, blue will be the predominant colour. The other colours of the
spectrum must, to some extent, be associated with the blue. They are
not absent, but deficient. We ought, in fact, to have them all, but
in diminishing proportions, from the violet to the red.
We have here presented a case to the imagination, pad, assuming the
undulatory theory to be a reality, we have, I think, fairly reasoned
our way to the conclusion, that were particles, small in comparison to
the sizes of the aether waves, sown in our atmosphere, the light
scattered by those particles would be exactly such as we observe in
our azure skies. When this light is analysed, all the colours of the
spectrum are found, and they are found in the proportions indicated by
our conclusion. Blue is not the sole, but it is the predominant
colour.
Let us now turn our attention to the light which passes unscattered
among the particles. How must it be finally affected? By its
successive collisions with the particles the white light is more and
more robbed of its shorter waves; it therefore loses more and more of
its due proportion of blue. The result may be anticipated. The
transmitted light, where short distances are involved, will appear
yellowish. But as the sun sinks towards the horizon the atmospheric
distances increase, and consequently the number of the scattering
particles. They abstract in succession the violet, the indigo, the
blue, and even disturb the proportions of green. The transmitted
light under such circumstances must pass from yellow through orange to
red. This also is exactly what we find in nature. Thus, while the
reflected light gives us at noon the deep azure of the Alpine skies,
the transmitted light gives us at sunset the warm crimson of the
Alpine snows. The phenomena certainly occur as if our atmosphere were
a medium rendered slightly turbid by the mechanical suspension of
exceedingly small foreign particles.
Here, as before, we encounter our sceptical 'as if.' It is one of the
parasites of science, ever at hand, and ready to plant itself and
sprout, if it can, on the weak points of our philosophy. But a strong
constitution defies the parasite, and in our case, as we question the
phenomena, probability grows like growing health, until in the end the
malady of doubt is completely extirpated. The first question that
naturally arises is this: Can small particles be really proved to act
in the manner indicated? No doubt of it. Each one of you can submit
the question to an experimental test. Water will not dissolve resin,
but spirit will dissolve it; and when spirit holding resin in solution
is dropped into water, the resin immediately separates in solid
particles, which render the water milky. The coarseness of this
precipitate depends on the quantity of the dissolved resin. You can
cause it to separate either in thick clots or in exceedingly fine
particles. Professor Bruecke has given us the proportions which
produce particles particularly suited to our present purpose. One
gramme of clean mastic is dissolved in eighty-seven grammes of
absolute alcohol, and the transparent solution is allowed to drop into
a beaker containing clear water, kept briskly stirred. An exceedingly
fine precipitate is thus formed, which declares its presence by its
action upon light. Placing a dark surface behind the beaker, and
permitting the light to fall into it from the top or front, the medium
is seen to be distinctly blue. It is not perhaps so perfect a blue as
may be seen on exceptional days among the Alps, but it is a very fair
sky-blue. A trace of soap in water gives a tint of blue. London, and
I fear Liverpool, milk makes an approximation to the same colour,
through the operation of the same cause; and Helmholtz has
irreverently disclosed the fact that the deepest blue eye is simply a
turbid medium.
*****
The action of turbid media upon light was illustrated by Goethe, who,
though unacquainted with the undulatory theory, was led by his
experiments to regard the firmament as an illuminated turbid medium,
with the darkness of space behind it. He describes glasses showing a
bright yellow by transmitted, and a beautiful blue by reflected,
light. Professor Stokes, who was probably the first to discern the
real nature of the action of small particles on the waves of aether,
[Footnote: This is inferred from conversation. I am not aware that
Professor Stokes has published anything upon the subject.] describes
a glass of a similar kind. [Footnote: This glass, by reflected light,
had a colour 'strongly resembling that of a decoction of
horse-chestnut bark.' Curiously enough, Goethe refers to this very
decoction: 'Man nehme einen Streifen frischer Rinds von der
Rosskastanie, man stecke denselben in ein Glas Wasser, und in der
kuerzesten Zeit werden wir das vollkommenste Himmelblau entstehen
sehen.'--Goethe's Werke, B. xxix. p. 24.]
Capital specimens of such glass are to be found at Salviati's, in St.
James's Street. What artists call 'chill' is no doubt an effect of
this description. Through the action of minute particles, the browns
of a picture often present the appearance of the bloom of a plum. By
rubbing the varnish with a silk handkerchief optical continuity is
established and the chill disappears. Some years ago I witnessed Mr.
Hirst experimenting at Zermatt on the turbid water of the Visp. When
kept still for a day or so, the grosser matter sank, but the finer
particles remained suspended, and gave a distinctly blue tinge to the
water. The blueness of certain Alpine lakes has been shown to be in
part due to this cause. Professor Roscoe has noticed several striking
cases of a similar kind. In a very remarkable paper the late
Principal Forbes showed that steam issuing from the safety-valve of a
locomotive, when favourably observed, exhibits at a certain stage of
its condensation the colours of the sky. It is blue by reflected
light, and orange or red by transmitted light. The same effect, as
pointed out by Goethe, is to some extent exhibited by peat-smoke. More
than ten years ago, I amused myself by observing, on a calm day at
Killarney, the straight smoke-columns rising from the cabin-chimneys.
It was easy to project the lower portion of a column against a dark
pine, and its upper portion against a bright cloud. The smoke in the
former case was blue, being seen mainly by reflected light; in the
latter case it was reddish, being seen mainly by transmitted light.
Such smoke was not in exactly the condition to give us the glow of the
Alps, but it was a step in this direction. Bruecke's fine precipitate
above referred to looks yellowish by transmitted light; but, by duly
strengthening the precipitate, you may render the white light of noon
as ruby-coloured as the sun, when seen through Liverpool smoke, or
upon Alpine horizons. I do not, however, point to the gross smoke
arising from coal as an illustration of the action of small particles,
because such smoke soon absorbs and destroys the waves of blue,
instead of sending them to the eyes of the observer.
These multifarious facts, and numberless others which cannot now be
referred to, are explained by reference to the single principle, that,
where the scattering particles are small in comparison to the
aethereal waves, we have in the reflected light a greater proportion
of the smaller waves, and in the transmitted light a greater
proportion of the larger waves, than existed in the original white
light. The consequence, as regards sensation, is that in the one ease
blue is predominant, and in the other orange or red. Our best
microscopes can readily reveal objects not more than 1/50000th of an
inch in diameter. This is less than the length of a wave of red
light. Indeed a first-rate microscope would enable us to discern
objects not exceeding in diameter the length of the smallest waves of
the visible spectrum. [Footnote: Dallinger and Drysdale have recently
measured cilia 1/200000th of an inch in diameter. 1878.] By the
microscope, therefore, we can test our particles. If they be as large
as the light-waves they will infallibly be seen; and if they be not so
seen, it is because they are smaller. Some months ago I placed in the
hands of our President a liquid containing Bruecke's precipitate. The
liquid was milky blue, and Mr. Huxley applied to it his highest
microscopic power. He satisfied me that had particles of even
1/100000th of an inch in diameter existed in the liquid, they could
not have escaped detection. But no particles were seen. Under the
microscope the turbid liquid was not to be distinguished from
distilled water. [Footnote: Like Dr. Burdon Sanderson's 'pyrogen,'
the particles of mastic passed, without sensible hindrance, through
filtering-paper. By such filtering no freedom from suspended
particles is secured. The application of a condensed beam to the
filtrate renders this at once evident.]
But we have it in our power to imitate, far more closely than we have
hitherto done, the natural conditions of this problem. We can
generate, in air, artificial skies, and prove their perfect identity
with the natural one, as regards the exhibition of a number of wholly
unexpected phenomena. By a continuous process of growth, moreover, we
are able to connect sky-matter, if I may use the term, with molecular
matter on the one side, and with molar matter, or matter in sensible
masses, on the other. In illustration of this, I will take an
experiment suggested by some of my own researches, and described by M.
Morren of Marseilles at the Exeter meeting of the British Association.
Sulphur and oxygen combine to form sulphurous acid gas, two atoms of
oxygen and one of sulphur constituting the molecule of sulphurous
acid. It has been recently shown that waves of aether issuing from a
strong source, such as the sun or the electric light, are competent to
shake asunder the atoms of gaseous molecules. [Footnote: See 'New
Chemical Reactions produced by Light,' vol. i.] A chemist would
call this, 'decomposition' by light; but it behoves us, who are
examining the power and function of the imagination, to keep
constantly before us the physical images which underlie our terms.
Therefore I say, sharply and definitely, that the components of the
molecules of sulphurous acid are shaken asunder by the aether-waves.
Enclosing sulphurous acid in a suitable vessel, placing it in a dark
room, and sending through it a powerful beam of light, we at first see
nothing: the vessel containing the gas seems as empty as a vacuum.
Soon, however, along the track of the beam a beautiful sky-blue colour
is observed, which is due to light scattered by the liberated
particles of sulphur. For a time the blue grows more intense; it then
becomes whitish; and ends in a more or less perfect white. When the
action is continued long enough, the tube is filled with a dense cloud
of sulphur particles, which by the application of proper means may be
rendered individually visible. [Footnote: M. Morren was mistaken in
supposing that a modicum of sulphurous acid, in the drying tubes, had
any share in the production of the 'actinic clouds' described by me. A
beautiful case of molecular instability in the presence of light is
furnished by peroxide of chlorine as proved by Professor Dewar. 1878.]
Here, then, our aether-waves untie the bond of chemical affinity, and
liberate a body--sulphur--which at ordinary temperatures is a solid,
and which therefore soon becomes an object of the senses. We have
first of all the free atoms of sulphur, which are incompetent to stir
the retina sensibly with scattered light. But these atoms gradually
coalesce and form _particles_, which grow larger by continual accretion,
until after a minute or two they appear as sky-matter. In this
condition they are individually invisible; but collectively they send
an amount of wave-motion to the retina, sufficient to produce the
firmamental blue. The particles continue, or may be caused to
continue, in this condition for a considerable time, during which no
microscope can cope with them. But they grow slowly larger, and pass
by insensible gradations into the state of _cloud_, when they can no
longer elude the armed eye. Thus, without solution of continuity, we
start with matter in the atom, and end with matter in the mass;
sky-matter being the middle term of the series of transformations.
Instead of sulphurous acid, we might choose a dozen other substances,
and produce the same effect with all of them. In the case of
some--probably in the case of all--it is possible to preserve matter
in the firmamental condition for fifteen or twenty minutes under the
continual operation of the light. During these fifteen or twenty
minutes the particles constantly grow larger, without ever exceeding
the size requisite to the production of the celestial blue.
Now when two vessels are placed before us, each containing sky-matter,
it is possible to state with great distinctness which vessel contains
the largest particles. The eye is very sensitive to differences of
light, when, as in our experiments, it is placed in comparative
darkness, and the wave-motion thrown against the retina is small. The
larger particles declare themselves by the greater whiteness of their
scattered light. Call now to mind the observation, or effort at
observation, made by our President, when he failed to distinguish the
particles of mastic in Bruecke's medium, and when you have done this,
please follow me.
A beam of light is permitted to act upon a certain vapour. In two
minutes the azure appears, but at the end of fifteen minutes it has
not ceased to be azure. After fifteen minutes its colour, and some
other phenomena, pronounce it to be a blue of distinctly smaller
particles than those sought for in vain by Mr. Huxley. These
particles, as already stated, must have been less than 1/100000th of
an inch in diameter.
And now I want you to consider the following question: Here are
particles which have been growing continually for fifteen minutes, and
at the end of that time are demonstrably smaller than those which
defied the microscope of Mr. Huxley--_What must have been the size of
these particles at the beginning of their growth?_ What notion can you
form of the magnitude of such particles? The distances of stellar
space give us simply a bewildering sense of vastness, without leaving
any distinct impression on the mind; and the magnitudes with which we
have here to do, bewilder us equally in the opposite direction. We
are dealing with infinitesimals, compared with which the test objects
of the microscope are literally immense.