over the phenomena exhibited by crystals in polarized light, in order
to give you some notion of the firmness and completeness of the theory
which grasps them all. Starting from the single assumption of
transverse undulations, we first of all determine the wave-lengths,
and find that on them all the phenomena of colour are dependent. The
wavelengths may be determined in many independent ways. Newton
virtually determined them when he measured the periods of his Fits:
the length of a fit, in fact, is that of a quarter of an undulation.
The wave-lengths may be determined by diffraction at the edges of a
slit (as in the Appendix to these Lectures); they may be deduced from
the interference fringes produced by reflection; from the fringes
produced by refraction; also by lines drawn with a diamond upon glass
at measured distances asunder. And when the length determined by these
independent methods are compared together, the strictest agreement is
found to exist between them.
With the wave-lengths once at our disposal, we follow the ether into
the most complicated cases of interaction between it and ordinary
matter, 'the theory is equal to them all. It makes not a single new
physical hypothesis; but out of its original stock of principles it
educes the counterparts of all that observation shows. It accounts
for, explains, simplifies the most entangled cases; corrects known
laws and facts; predicts and discloses unknown ones; becomes the guide
of its former teacher Observation; and, enlightened by mechanical
conceptions, acquires an insight which pierces through shape and
colour to force and cause.'[18]
But, while I have thus endeavoured to illustrate before you the power
of the undulatory theory as a solver of all the difficulties of
optics, do I therefore wish you to close your eyes to any evidence
that may arise against it? By no means. You may urge, and justly urge,
that a hundred years ago another theory was held by the most eminent
men, and that, as the theory then held had to yield, the undulatory
theory may have to yield also. This seems reasonable; but let us
understand the precise value of the argument. In similar language a
person in the time of Newton, or even in our time, might reason thus:
Hipparchus and Ptolemy, and numbers of great men after them, believed
that the earth was the centre of the solar system. But this deep-set
theoretic notion had to give way, and the helio-centric theory may, in
its turn, have to give way also. This is just as reasonable as the
first argument. Wherein consists the strength of the present theory of
gravitation? Solely in its competence to account for all the phenomena
of the solar system. Wherein consists the strength of the theory of
undulation? Solely in its competence to disentangle and explain
phenomena a hundred-fold more complex than those of the solar system.
Accept if you will the scepticism of Mr. Mill[19] regarding the
undulatory theory; but if your scepticism be philosophical, it will
wrap the theory of gravitation in the same or in greater doubt.[20]
§ 11. _The Blue of the Sky_.
I am unwilling to quit these chromatic phenomena without referring to
a source of colour which has often come before me of late in the blue
of your skies at noon, and the deep crimson of your horizon after the
set of sun. I will here summarize and extend what I have elsewhere
said upon this subject. Proofs of the most cogent description could be
adduced to show that the blue light of the firmament is reflected
light. That light 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. The
solar light, moreover, is not scattered by the sky in the proportions
which produce white. The sky is blue, which indicates an excess of the
smaller waves. The blueness of the air has been given as a reason for
the blueness of the sky; but then the question arises, 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
the white solar light through a blue medium could by no possibility
redden the light; the hypothesis of a blue atmosphere is therefore
untenable. In fact, the agent, whatever it be, 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 reflection from the sky and that
from an ordinary cloud, which exercises no such dichroitic action.
The cloud, in fact, takes no note of size on the part of the waves of
ether, but reflects them all alike. Now the cause of this may be that
the cloud-particles are so large in comparison with the size of the
waves of ether as to scatter them all indifferently. A broad cliff
reflects an Atlantic roller as easily as it reflects a ripple produced
by a sea-bird's wing; and, in the presence of large reflecting
surfaces, the existing differences of magnitude among the waves of
ether may also disappear. But supposing the reflecting particles,
instead of being very large, to be very small, in comparison with the
size of the waves. Then, instead of the whole wave being fronted and
in great part thrown back, a small portion only is shivered off by the
obstacle. Suppose, then, such minute foreign particles to be diffused
in our atmosphere. Waves of all sizes impinge upon them, and at every
collision a portion of the impinging wave is struck off. All the waves
of the spectrum, from the extreme red to the extreme violet, are thus
acted upon; but in what proportions will they be scattered? 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 relative reflection.
A small pebble, placed in the way of the ring-ripples produced by
heavy rain-drops on a tranquil pond, will throw back a large fraction
of each ripple incident upon it, while the fractional part of a larger
wave thrown back by the same pebble might be infinitesimal. Now to
preserve the solar light white, its constituent proportions must not
be altered; but in the scattering of the light by these very small
particles we see that the proportions _are_ altered. The smaller waves
are in excess, 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 thus reasoned our way to the conclusion, that were particles,
small in comparison to the size of the ether waves, sown in our
atmosphere, the light scattered by those particles would be exactly
such as we observe in our azure skies. And, indeed, when this light is
analyzed, all the colours of the spectrum are found in the proportions
indicated by our conclusion.
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 moderate distances are involved, will appear
yellowish. But as the sun sinks towards the horizon the atmospheric
distance increases, and consequently the number of the scattering
particles. They weaken 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.
But 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;
and when spirit which holds 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. Professor Brücke 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 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 of a very fair sky-blue. A trace of
soap in water gives it a tint of blue. London milk makes an
approximation to the same colour, through the operation of the same
cause: and Helmholtz has irreverently disclosed the fact that a blue
eye is simply a turbid medium.
§ 12. _Artificial Sky_.
But we have it in our power to imitate far more closely 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. It has been
recently shown in a great number of instances by myself that waves of
ether issuing from a strong source, such as the sun or the electric
light, are competent to shake asunder the atoms of gaseous molecules.
The apparatus used to illustrate this consists of a glass tube about a
yard in length, and from 2½ to 3 inches internal diameter. The gas or
vapour to be examined is introduced into this tube, and upon it the
condensed beam of the electric lamp is permitted to act. The vapour is
so chosen that one, at least, of its products of decomposition, as
soon as it is formed, shall be _precipitated_ to a kind of cloud. By
graduating the quantity of the vapour, this precipitation may be
rendered of any degree of fineness, forming particles distinguishable
by the naked eye, or particles which are probably far beyond the reach
of our highest microscopic powers. I have no reason to doubt that
particles may be thus obtained whose diameters constitute but a very
small fraction of the length of a wave of violet light.
Now, in all such cases when suitable vapours are employed in a
sufficiently attenuated state, no matter what the vapour may be, the
visible action commences with the formation of a _blue cloud_. Let me
guard myself at the outset against all misconception as to the use of
this term. The blue cloud here referred to is totally invisible in
ordinary daylight. To be seen, it requires to be surrounded by
darkness, _it only_ being illuminated by a powerful beam of light.
This cloud differs in many important particulars from the finest
ordinary clouds, and might justly have assigned to it an intermediate
position between these clouds and true cloudless vapour.
It is possible to make the particles of this _actinic cloud_ grow from
an infinitesimal and altogether ultra-microscopic size to particles of
sensible magnitude; and by means of these in a certain stage of their
growth, we produce a blue which rivals, if it does not transcend, that
of the deepest and purest Italian sky. Introducing into our tube a
quantity of mixed air and nitrite of butyl vapour sufficient to
depress the mercurial column of an air-pump one-twentieth of an inch,
adding a quantity of air and hydrochloric acid sufficient to depress
the mercury half an inch further, and sending through this compound
and highly attenuated atmosphere the beam of the electric light,
within the tube arises gradually a splendid azure, which strengthens
for a time, reaches a maximum of depth and purity, and then, as the
particles grow larger, passes into whitish blue. This experiment is
representative, and it illustrates a general principle. Various other
colourless substances of the most diverse properties, optical and
chemical, might be employed for this experiment. The _incipient
cloud_, in every case, would exhibit this superb blue; thus proving to
demonstration that particles of infinitesimal size, without any colour
of their own, and irrespective of those optical properties exhibited
by the substance in a massive state, are competent to produce the blue
colour of the sky.
§ 13. _Polarization of Skylight_.
But there is another subject connected with our firmament, of a more
subtle and recondite character than even its colour. I mean that
'mysterious and beautiful phenomenon,' as Sir John Herschel calls it,
the polarization of the light of the sky. Looking at various points of
the blue firmament through a Nicol prism, and turning the prism round
its axis, we soon notice variations of brightness. In certain
positions of the prism, and from certain points of the firmament, the
light appears to be wholly transmitted, while it is only necessary to
turn the prism round its axis through an angle of ninety degrees to
materially diminish the intensity of the light. Experiments of this
kind prove that the blue light sent to us by the firmament is
polarized, and on close scrutiny it is also found that the direction
of most perfect polarization is perpendicular to the solar rays. Were
the heavenly azure like the ordinary light of the sun, the turning of
the prism would have no effect upon it; it would be transmitted
equally during the entire rotation of the prism. The light of the sky
may be in great part quenched, because it is in great part polarized.
The same phenomenon is exhibited in perfection by our actinic clouds,
the only condition necessary to its production being the smallness of
the particles. In all cases, and with all substances, the cloud formed
at the commencement, when the precipitated particles are sufficiently
fine, is _blue_. In all cases, moreover, this fine blue cloud
polarizes _perfectly_ the beam which illuminates it, the direction of
polarization enclosing an angle of 90° with the axis of the
illuminating beam.
It is exceedingly interesting to observe both the growth and the decay
of this polarization. For ten or fifteen minutes after its first
appearance, the light from a vividly illuminated incipient cloud,
looked at horizontally, is absolutely quenched by a Nicol prism with
its longer diagonal vertical. But as the sky-blue is gradually
rendered impure by the introduction of particles of too large a size,
in other words, as real clouds begin to be formed, the polarization
begins to deteriorate, a portion of the light passing through the
prism in all its positions, as it does in the case of skylight. It is
worthy of note that for some time after the cessation of perfect
polarization 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. 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 light discharged at right angles to the beam.
The perfection of the polarization in a direction perpendicular to the
illuminating beam may be also illustrated by the following experiment,
which has been executed with many vapours. A Nicol prism large enough
to embrace the entire beam of the electric lamp was placed between the
lamp and the experimental tube. Sending the beam polarized by the
Nicol through the tube, I placed myself in front of it, the eyes 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 a vivid blue cloud was seen when looked at
downwards or upwards. This truly fine experiment, which I should
certainly have made without suggestion, was, as a matter of fact,
first definitely suggested by a remark addressed to me in a letter by
Professor Stokes.
All the phenomena of colour and of polarization observable in the case
of skylight are manifested by those actinic clouds; and they exhibit
additional phenomena which it would be neither convenient to pursue,
nor perhaps possible to detect, in the actual firmament. They enable
us, for example, to follow the polarization from its first appearance
on the barely visible blue to its final extinction in the coarser
cloud. These changes, as far as it is now necessary to refer to them,
may be thus summed up:--
1. The actinic cloud, as long as it continues blue, discharges
polarized light in all directions, but the direction of maximum
polarization, like that of skylight, is at right angles to the
direction of the illuminating beam.
2. As long as the cloud remains distinctly blue, the light discharged
from it at right angles to the illuminating beam is _perfectly_
polarized. It may be utterly quenched by a Nicol prism, the cloud from
which it issues being caused to disappear. Any deviation from the
perpendicular enables a portion of the light to get through the prism.
3. The direction of vibration of the polarized light is at right
angles to the illuminating beam. Hence a plate of tourmaline, with its
axis parallel to the beam, stops the light, and with the axis
perpendicular to the beam transmits the light.
4. A plate of selenite placed between the Nicol and the actinic cloud
shows the colours of polarized light; in fact, the cloud itself plays
the part of a polarizing Nicol.
5. The particles of the blue cloud are immeasurably small, but they
increase gradually in size, and at a certain period of their growth
cease to discharge perfectly polarized light. For some time afterwards
the light that reaches the eye, through the Nicol in its position of
least transmission, is of a magnificent blue, far exceeding in depth
and purity that of the purest sky; thus the waves that first feel the
influence of size, at both limits of the polarization, are the
shortest waves of the spectrum. These are the first to accept
polarization, and they are the first to escape from it.
LECTURE V.
RANGE OF VISION NOT COMMENSURATE WITH RANGE OF RADIATION
THE ULTRA-VIOLET BAYS
FLUORESCENCE
THE RENDERING OF INVISIBLE RAYS VISIBLE
VISION NOT THE ONLY SENSE APPEALED TO BY THE SOLAR AND ELECTRIC BEAM
HEAT OF BEAM
COMBUSTION BY TOTAL BEAM AT THE FOCI OF MIRRORS AND LENSES
COMBUSTION THROUGH ICE-LENS
IGNITION OF DIAMOND
SEARCH FOR THE RAYS HERE EFFECTIVE
SIR WILLIAM HERSCHEL'S DISCOVERY OF DARK SOLAR RAYS
INVISIBLE RAYS THE BASIS OF THE VISIBLE
DETACHMENT BY A RAY-FILTER OF THE INVISIBLE RAYS FROM THE VISIBLE
COMBUSTION AT DARK FOCI
CONVERSION OF HEAT-RAYS INTO LIGHT-RAYS
CALORESCENCE
PART PLAYED IN NATURE BY DARK RAYS
IDENTITY OF LIGHT AND RADIANT HEAT
INVISIBLE IMAGES
REFLECTION, REFRACTION, PLANE POLARIZATION, DEPOLARIZATION,
CIRCULAR POLARIZATION, DOUBLE REFRACTION, AND MAGNETIZATION
OF RADIANT HEAT.
§ 1. _Range of Vision and of Radiation_.
The first question that we have to consider to-night is this: Is the
eye, as an organ of vision, commensurate with the whole range of solar
radiation--is it capable of receiving visual impressions from all the
rays emitted by the sun? The answer is negative. If we allowed
ourselves to accept for a moment that notion of gradual growth,
amelioration, and ascension, implied by the term _evolution_, we might
fairly conclude that there are stores of visual impressions awaiting
man, far greater than those now in his possession. Ritter discovered
in 1801 that beyond the extreme violet of the spectrum there is a vast
efflux of rays which are totally useless as regards our present powers
of vision. These ultra-violet waves, however, though incompetent to
awaken the optic nerve, can shake asunder the molecules of certain
compound substances on which they impinge, thus producing chemical
decomposition.
But though the blue, violet, and ultra-violet rays can act thus upon
certain substances, the fact is hardly sufficient to entitle them to
the name of 'chemical rays,' which is usually applied to distinguish
them from the other constituents of the spectrum. As regards their
action upon the salts of silver, and many other substances, they may
perhaps merit this title; but in the case of the grandest example of
the chemical action of light--the decomposition of carbonic acid in
the leaves of plants, with which my eminent friend Dr. Draper (now no
more) has so indissolubly associated his name--the yellow rays are
found to be the most active.
There are substances, however, on which the violet and ultra-violet
waves exert a special decomposing power; and, by permitting the
invisible spectrum to fall upon surfaces prepared with such
substances, we reveal both the existence and the extent of the
ultraviolet spectrum.
§ 2. _Ultra-violet Rays: Fluorescence_.
The method of exhibiting the action of the ultraviolet rays by their
chemical action has been long known; indeed, Thomas Young photographed
the ultra-violet rings of Newton. We have now to demonstrate their
presence in another way. As a general rule, bodies either transmit
light or absorb it; but there is a third case in which the light
falling upon the body is neither transmitted nor absorbed, but
converted into light of another kind. Professor Stokes, the occupant
of the chair of Newton in the University of Cambridge, has
demonstrated this change of one kind of light into another, and has
pushed his experiments so far as to render the invisible rays visible.
A large number of substances examined by Stokes, when excited by the
invisible ultra-violet waves, have been proved to emit light. You know
the rate of vibration corresponding to the extreme violet of the
spectrum; you are aware that to produce the impression of this colour,
the retina is struck 789 millions of millions of times in a second. At
this point, the retina ceases to be useful as an organ of vision; for,
though struck by waves of more rapid recurrence, they are incompetent
to awaken the sensation of light. But when such non-visual waves are
caused to impinge upon the molecules of certain substances--on those
of sulphate of quinine, for example--they compel those molecules, or
their constituent atoms, to vibrate; and the peculiarity is, that the
vibrations thus set up are _of slower period_ than those of the
exciting waves. By this lowering of the rate of vibration through the
intermediation of the sulphate of quinine, the invisible rays are
brought within the range of vision. We shall subsequently have
abundant opportunity for learning that transparency to the visible by
no means involves transparency to the invisible rays. Our bisulphide
of carbon, for example, which, employed in prisms, is so eminently
suitable for experiments on the visual rays, is by no means so
suitable for these ultra-violet rays. Flint glass is better, and rock
crystal is better than flint glass. A glass prism, however, will suit
our present purpose.