until the experimental tube, which had been previously exhausted, was
filled with the mixed air and vapour. The visible action of light
upon the mixture after fifteen minutes' exposure was slight. The tube
was afterwards filled with half an atmosphere of the mixed air and
vapour, and a second half-atmosphere of air which had been permitted
to bubble through fresh commercial hydrochloric acid. On sending the
beam through this mixture, the tube, for a moment, was optically
empty. But the pause amounted only to a small fraction of a second, a
dense cloud being immediately precipitated upon the beam.
This cloud began blue, but the advance to whiteness was so rapid as
almost to justify the application of the term instantaneous. The
dense cloud, looked at perpendicularly to its axis, showed scarcely
any signs of polarisation. Looked at obliquely the polarisation was
strong.
The experimental tube being again cleansed and exhausted, the mixed
air and nitrite-of-butyl vapour was permitted to enter it until the
associated mercury column was depressed 1/10 of an inch. In other
words, the air and vapour, united, exercised a pressure not exceeding
1/300th of an atmosphere. Air, passed through a solution of
hydrochloric acid, was then added, till the mercury column was
depressed three inches. The condensed beam of the electric light was
passed for some time through this mixture without revealing anything
within the tube competent to scatter the light. Soon, however, a
superbly blue cloud was formed along, the track of the beam, and it
continued blue sufficiently long to permit of its thorough
examination. The light discharged from the cloud, at right angles to
its own length, was at first perfectly polarised. It could be totally
quenched by the Nicol. By degrees the cloud became of whitish blue,
and for a time the selenite colours, obtained by looking at it
normally, were exceedingly brilliant. The direction of maximum
polarisation was distinctly at right angles to the illuminating beam.
This continued to be the case as long as the cloud maintained a
decided blue colour, and even for some time after the blue had changed
to whitish blue. But, as the light continued to act, the cloud became
coarser and whiter, particularly at its centre, where it at length
ceased to discharge polarised light in the direction of the
perpendicular, while it continued to do so at both ends.
But the cloud which had thus ceased to polarise the light emitted
normally, showed vivid selenite colours when looked at obliquely,
proving that the direction of maximum polarisation changed with the
texture of the cloud. This point shall receive further illustration
subsequently.
A blue, equally rich and more durable, was obtained by employing the
nitrite-of-butyl vapour in a still more attenuated condition. The
instance here cited is representative. In all cases, and with all
substances, the cloud formed at the commencement, when the
precipitated particles are sufficiently fine, is _blue_, and it can be
made to display a colour rivalling that of the purest Italian sky. In
all cases, moreover, this fine blue cloud polarises _perfectly_ the beam
which illuminates it, the direction of polarisation enclosing an angle
of 90° with the axis of the illuminating beam.
It is exceedingly interesting to observe both the perfection and the
decay of this polarisation. For ten or fifteen minutes after its
first appearance the light from a vividly illuminated actinic cloud,
looked at perpendicularly, is absolutely quenched by a Nicol's prism
with its longer diagonal vertical. But as the sky-blue is gradually
rendered impure by the growth of the particles--in other words, as
real clouds begin to be formed--the polarisation begins to decay, a
portion of the light passing through the prism in all its positions.
It is worthy of note, that for some time after the cessation of
perfect polarisation, the residual light which passes, when the Nicol
is in its position of minimum transmission, is of a gorgeous blue, the
whiter light of the cloud being extinguished. [Footnote: This shows
that particles too large to polarise the blue, polarise perfectly
light of lower refrangibility.] When the cloud texture has become
sufficiently coarse to approximate to that of ordinary clouds, the
rotation of the Nicol ceases to have any sensible effect on the
quantity of light discharged normally.
The perfection of the polarisation, in a direction perpendicular to
the illuminating beam, is also illustrated by the following
experiments: A Nicol's prism, large enough to embrace the entire beam
of the electric lamp, was placed between the lamp and the experimental
tube. A few bubbles of air, carried through the liquid nitrite of
butyl, were introduced into the tube, and they were followed by about
three inches (measured by the mercurial gauge) of air which had passed
through aqueous hydrochloric acid. Sending the polarised beam through
the tube, I placed myself in front of it, my eye being on a level with
its axis, my assistant occupying a similar position behind the tube.
The short diagonal of the large Nicol was in the first instance
vertical, the plane of vibration of the emergent beam being therefore
also vertical. As the light continued to act, a superb blue cloud,
visible to both my assistant and myself, was slowly formed. But this
cloud, so deep and rich when looked at from the positions mentioned,
_utterly disappeared when looked at vertically downwards, or vertically
upwards_. Reflection from the cloud was not possible in these
directions. When the large Nicol was slowly turned round its axis,
the eye of the observer being on the level of the beam, and the line
of vision perpendicular to it, entire extinction of the light emitted
horizontally occurred when the longer diagonal of the large Nicol was
vertical. But now a vivid blue cloud was seen when looked at
downwards or upwards. This truly fine experiments, which I
contemplated making on my own account, was first definitely suggested
by a remark in a letter addressed to me by Professor Stokes.
As regards the polarisation of skylight, the greatest stumbling-block
has hitherto been, that, in accordance with the law of Brewster, which
makes the index of refraction the tangent of the polarising angle, the
reflection which produces perfect polarisation would require to be
made in air upon air; and indeed this led many of our most eminent
men, Brewster himself among the number, to entertain the idea of
aerial molecular reflection. [Footnote: 'The cause of the
polarisation is evidently a reflection of the sun's light upon
something. The question is on what? Were the angle of maximum
polarisation 76°, we should look to water or ice as the reflecting
body, however inconceivable the existence in a cloudless atmosphere
and a hot summer's day of unevaporated molecules (particles?) of
water. But though we were once of this opinion, careful observation
has satisfied us that 90°, or thereabouts, is the correct angle, and
that therefore whatever be the body on which the light has been
reflected, if polarised by a single reflection, the polarising angle
must be 45°, and the index of refraction, which is the tangent of that
angle, unity; in other words, the reflection would require to be made
in air upon air!' (Sir John Herschel, 'Meteorology,' par. 233.)
Any particles, if small enough, will produce both the colour and the
polarisation of the sky. But is the existence of small
water-particles on a hot summer's day in the higher regions of our
atmosphere inconceivable? It is to be remembered that the oxygen and
nitrogen of the air behave as a vacuum to radiant heat, the
exceedingly attenuated vapour of the higher atmosphere being therefore
in practical contact with the cold of space.]
I have, however, operated upon substances of widely different
refractive indices, and therefore of very different polarising angles
as ordinarily defined, but the polarisation of the beam, by the
incipient cloud, has thus far proved itself to be absolutely
independent of the polarising angle. The law of Brewster does not
apply to matter in this condition, and it rests with the undulatory
theory to explain why. Whenever the precipitated particles are
sufficiently fine, no matter what the substance forming the particles
may be, the direction of maximum polarisation is at right angles to
the illuminating beam, the polarising angle for matter in this
condition being invariably 45°.
Suppose our atmosphere surrounded by an envelope impervious to light,
but with an aperture on the sunward side through which a parallel beam
of solar light could enter and traverse the atmosphere. Surrounded by
air not directly illuminated, the track of such a beam would resemble
that of the parallel beam of the electric lamp through an incipient
cloud. The sunbeam would be blue, and it would discharge laterally
light in precisely the same condition as that discharged by the
incipient cloud. In fact, the azure revealed by such a beam would be
to all intents and purposes that which I have called a 'blue cloud.'
Conversely our 'blue cloud' is, to all intents and purposes, an
_artificial sky_.' [Footnote: The opinion of Sir John Herschel,
connecting the polarisation and the blue colour of the sky, is
verified by the foregoing results. 'The more the subject [the
polarisation of skylight] is considered,' writes this eminent
philosopher, 'the more it will be found beset with difficulties, and
its explanation when arrived at will probably be found to carry with
it that of the blue colour of the sky itself, and of the great
quantity of light it actually does send down to us.' 'We may observe,
too,' he adds, 'that it is only where the purity of the sky is most
absolute that the polarisation is developed in its highest degree, and
that where there is the slightest perceptible tendency to cirrus it is
materially impaired.' This applies word for word to our 'incipient
clouds.']
But, as regards the polarisation of the sky, we know that not only is
the direction of maximum polarisation at right angles to the track of
the solar beams, but that at certain angular distances, probably
variable ones, from the sun, 'neutral points,' or points of no
polarisation, exist, on both sides of which the planes of atmospheric
polarisation are at right angles to each other. I have made various
observations upon this subject which are reserved for the present;
but, pending the more complete examination of the question, the
following facts bearing upon it may be submitted.
The parallel beam employed in these experiments tracked its way
through the laboratory air, exactly as sunbeams are seen to do in the
dusty air of London. I have reason to believe that a great portion of
the matter thus floating in the laboratory air consists of organic
particles, which are capable of imparting a perceptibly bluish tint to
the air. These also showed, though far less vividly, all the effects
of polarisation obtained with the incipient clouds. The light
discharged laterally from the track of the illuminating beam was
polarised, though not perfectly, the direction of maximum polarisation
being at right angles to the beam. At all points of the beam,
moreover, throughout its entire length, the light emitted normally was
in the same state of polarisation. Keeping the positions of the Nicol
and the selenite constant, the same colours were observed throughout
the entire beam, when the line of vision was perpendicular to its
length.
The horizontal column of air, thus illuminated, was 18 feet long, and
could therefore be looked at very obliquely. I placed myself near the
end of the beam, as it issued from the electric lamp, and, looking
through the Nicol and selenite more and more obliquely at the beam,
observed the colours fading until they disappeared. Augmenting the
obliquity the colours appeared once more, but they were now
complementary to the former ones.
Hence this beam, like the sky, exhibited a neutral point, on opposite
sides of which the light was polarised in planes at right angles to
each other.
Thinking that the action observed in the laboratory might be caused,
in some way, by the vaporous fumes diffused in its air, I had the
light removed to a room at the top of the Royal Institution. The
track of the beam was seen very finely in the air of this room, a
length of 14 or 15 feet being attainable. This beam exhibited all the
effects observed with the beam in the laboratory. Even the
uncondensed electric light falling on the floating matter showed,
though faintly, the effects of polarisation.
When the air was so sifted as to entirely remove the visible floating
matter, it no longer exerted any sensible action upon the light, but
behaved like a vacuum. The light is scattered and polarised by
_particles_, not by molecules or atoms.
By operating upon the fumes of chloride of ammonium, the smoke of
brown paper, and tobacco-smoke, I had varied and confirmed in many
ways those experiments on neutral points, when my attention was drawn
by Sir Charles Wheatstone to an important observation communicated to
the Paris Academy in 1860 by Professor Govi, of Turin. [Footnote:
Comptes Rendus,' tome li, pp. 360 and 669.] M. Govi had been led to
examine a beam of light sent through a room in which were successively
diffused the smoke of incense, and tobacco-smoke. His first brief
communication stated the fact of polarisation by such smoke; but in
his second communication he announced the discovery of a neutral point
in the beam, at the opposite sides of which the light was polarised in
planes at right angles to each other.
But unlike my observations on the laboratory air, and unlike the
action of the sky, the direction of maximum polarisation in M. Govi's
experiments enclosed a very small angle with the axis of the
illuminating beam. The question was left in this condition, and I am
not aware that M. Govi or any other investigator has pursued it
further.
I had noticed, as before stated, that as the clouds formed in the
experimental tube became denser, the polarisation of the light
discharged at right angles to the beam became weaker, the direction of
maximum polarisation becoming oblique to the beam. Experiments on the
fumes of chloride of ammonium gave me also reason to suspect that the
position of the neutral point was not constant, but that it varied
with the density of the illuminated fumes.
The examination of these questions led to the following new and
remarkable results: The laboratory being well filled with the fumes of
incense, and sufficient time being allowed for their uniform
diffusion, the electric beam was sent through the smoke. From the
track of the beam polarised light was discharged; but the direction of
maximum polarisation, instead of being perpendicular, now enclosed an
angle of only 12° or 13° with the axis of the beam.
A neutral point, with complementary effects at opposite sides of it,
was also exhibited by the beam. The angle enclosed by the axis of the
beam, and a line drawn from the neutral point to the observer's eye,
measured in the first instance 66°.
The windows of the laboratory were now opened for some minutes, a
portion of the incense-smoke being permitted to escape. On again
darkening the room and turning on the light, the line of vision to the
neutral point was found to enclose, with the axis of the beam, an
angle of 63°.
The windows were again opened for a few minutes, more of the smoke
being permitted to escape. Measured as before, the angle referred to
was found to be 54°.
This process was repeated three additional times the neutral point was
found to recede lower and lower down the beam, the angle between a
line drawn from the eye to the neutral point and the axis of the beam
falling successively from 54° to 49°, 43° and 33°.
The distances, roughly measured, of the neutral point from the lamp,
corresponding to the foregoing series of observations, were these:
1st observation 2 feet 2 inches.
2nd observation 2 feet 6 inches.
3rd observation 2 feet 10 inches.
4th observation 3 feet 2 inches.
5th observation 3 feet 7 inches.
6th observation 4 feet 6 inches.
At the end of this series of experiments the direction of maximum
polarisation had again become normal to the beam.
The laboratory was next filled with the fumes of gunpowder. In five
successive experiments, corresponding to five different densities of
the gunpowder-smoke, the angles enclosed between the line of vision to
the neutral point and the axis of the beam, were 63 degrees, 50°, 47°,
42°, and 38° respectively.
After the clouds of gunpowder had cleared away, the laboratory was
filled with the fumes of common resin, rendered so dense as to be very
irritating to the lungs. The direction of maximum polarisation
enclosed, in this case, an angle of 12°, or thereabouts, with the axis
of the beam. Looked at, as in the former instances, from a position
near the electric lamp, no neutral point was observed throughout the
entire extent of the beam.
When this beam was looked at normally through the selenite and Nicol,
the ring-system, though not brilliant, was distinct. Keeping the eye
upon the plate of selenite, and the line of vision perpendicular, the
windows were opened, the blinds remaining undrawn. The resinous fumes
slowly diminished, and as they did so the ring-system became paler. It
finally disappeared. Continuing to look in the same direction, the
rings revived, but now the colours were complementary to the former
ones. _The neutral point had passed me in its motion down the beam,
consequent upon the attenuation of the fumes of resin_.
With the fumes of chloride of ammonium substantially the same results
were obtained. Sufficient, however, has been here stated to
illustrate the variability of the position of the neutral
point. [Footnote: Brewster has proved the variability of the position
of the neutral point for skylight with the sun's altitude, a result
obviously connected with the foregoing experiments.]
By a puff of tobacco-smoke, or of condensed steam, blown into the
illuminated beam, the brilliancy of the selenite colours may be
greatly enhanced. But with different clouds two different effects are
produced. Let the ring-system observed in the common air be brought
to its maximum strength, and then let an attenuated cloud of chloride
of ammonium be thrown into the beam at the point looked at; the ring
system flashes out with augmented brilliancy, but the character of the
polarisation remains unchanged. This is also the case when
phosphorus, or sulphur, is burned underneath the beam, so as to cause
the fine particles of phosphorus or of sulphur to rise into the light.
With the sulphur-fumes the brilliancy of the colours is exceedingly
intensified; but in none of these cases is there any change in the
character of the polarisation.
But when a puff of the fumes of hydrochloric acid, hydriodic acid, or
nitric acid is thrown into the beam, there is a complete reversal of
the selenite tints. Each of these clouds twists the plane of
polarisation 90°, causing the centre of the ring-system to change from
black to white, and the rings themselves to emit their complementary
colours. [Footnote: Sir John Herschel suggested to me that this
change of the polarisation from positive to negative may indicate a
change from polarisation by reflection to polarisation by refraction.
This thought repeatedly occurred to me while looking at the effects;
but it will require much following up before it emerges into
clearness.]
Almost all liquids have motes in them sufficiently numerous to
polarise sensibly the light, and very beautiful effects may be
obtained by simple artificial devices. When, for example, a cell of
distilled water is placed in front of the electric lamp, and a thin
slice of the beam is permitted to pass through it, scarcely any
polarised light is discharged, and scarcely any colour produced with a
plate of selenite. But if a bit of soap be agitated in the water above
the beam, the moment the infinitesimal particles reach the light the
liquid sends forth laterally almost perfectly polarised light; and if
the selenite be employed, vivid colours flash into existence. A still
more brilliant result is obtained with mastic dissolved in a great
excess of alcohol.
The selenite rings, in fact, constitute an extremely delicate test as
to the collective quantity of individually invisible particles in a
liquid. Commencing with distilled water, for example, a thick slice
of light is necessary to make the polarisation of its suspended
particles sensible. A much thinner slice suffices for common water;
while, with Bruecke's precipitated mastic, a slice too thin to produce
any sensible effect with most other liquids, suffices to bring out
vividly the selenite colours.
3. THE SKY OF THE ALPS.
The vision of an object always implies a differential action on the
retina of the observer. The object is distinguished from surrounding
space by its excess or defect of light in relation to that space. By
altering the illumination, either of the object itself or of its
environment, we alter the appearance of the object. Take the case of
clouds floating in the atmosphere with patches of blue between them.
Anything that changes the illumination of either alters the appearance
of both, that appearance depending, as stated, upon differential
action.
Now the light of the sky, being polarised, may, as the reader of the
foregoing pages knows, be in great part quenched by a Nicol's prism,
while the light of a common cloud, being unpolarised, cannot be thus
extinguished. Hence the possibility of very remarkable variations,
not only in the aspect of the firmament, which is really changed, but
also in the aspect of the clouds, which have that firmament as a
background. It is possible, for example, to choose clouds of such a
depth of shade that when the Nicol quenches the light behind them,
they shall vanish, being undistinguishable from the residual dull tint
which outlives the extinction of the brilliancy of the sky. A cloud
less deeply shaded, but still deep enough, when viewed with the naked
eye, to appear dark on a bright ground, is suddenly changed to a white
cloud on a dark ground by the quenching of the light behind it. When
a reddish cloud at sunset chances to float in the region of maximum
polarisation, the quenching of the surrounding light causes it to
flash with a brighter crimson. Last Easter eve the Dartmoor sky, which
had just been cleansed by a snow-storm, wore a very wild appearance.
Round the horizon it was of steely brilliancy, while reddish cumuli
and cirri floated southwards. When the sky was quenched behind them
these floating masses seemed like dull embers suddenly blown upon;
they brightened like a fire.
In the Alps we have the most magnificent examples of crimson clouds
and snows, so that the effects just referred to may be here studied
under the best possible conditions. On August 23, 1869, the evening
Alpenglow was very fine, though it did not reach its maximum depth and
splendour. The side of the Weisshorn seen from the Bel Alp, being
turned from the sun, was tinted mauve; but I wished to observe one of
the rose-coloured buttresses of the mountain. Such a one was visible
from a point a few hundred feet above the hotel. The Matterhorn also,
though for the most part in shade, had a crimson projection, while a
deep ruddy red lingered along its western shoulder. Four distinct
peaks and buttresses of the Dom, in addition to its dominant head--all
covered with pure snow--were reddened by the light of sunset. The
shoulder of the Alphubel was similarly coloured, while the great mass
of the Fletschorn was all a-glow, and so was the snowy spine of the
Monte Leone.