LIGHT.
Theories of the Nature of Light--Hypotheses of Newton
and Huygens--Sources of Light--The Sun--Velocity of
Light--Transparency--Dark Lines of the Spectrum--Absorption
of Light--Colour--Prismatic Analysis--Rays of the
Spectrum--Rainbow--Diffraction--Interference--Goethe’s
Theory--Polarisation--Magnetisation of Light--Vision--The
Eye--Analogy--Sound and Light--Influence of Light on Animals and
Vegetables--Phosphorescence arising from several Causes--Artificial
Light--Its Colour dependent on Matter.
Light, the first creation, presents to the enquiring mind a series
of phenomena of the most exalted character. The glowing sunshine,
painting the earth with all the brilliancy of colour, and giving to the
landscape the inimitable charm of every degree of illumination, from
the grey shadow to the golden glow;--the calm of evening, when, weary
of the “excess of splendour,” the eye can repose in tranquillity upon
the “cloud-land” of the west, and watch the golden and the ruddy hues
fade slowly into the blue tincture of night;--and the pale refulgence
of the moon, with the quiet sparkle of the sun-lit stars,--all tend
to impress upon the soul, the great truth that, where there is light,
organisation and life are found, and beyond its influence death and
silence hold supreme dominion.[84] Through all time we have evidences
that this has been the prevailing feeling of the human race, derived,
of course, from their observation of the natural phenomena dependent
upon luminous agency. In the myths of every country, impersonations of
light prevail, and to these are referred the mysteries of the perpetual
renewal of life on the surface of the earth.
This presentiment of a philosophic truth, in the instance of the poet
sages of intellectual Greece, was advanced to the highest degree of
refinement; and the sublime exclamation of Plato: “Light is truth, and
God is light,” approaches nearly to a divine revelation.
As the medium of vision--as the cause of colour--as a power influencing
in a most striking manner all the forms of organisation around us,
light presented to the inquiring minds of all ages a subject of the
highest interest.
The ancient philosophers, although they lost themselves in the
metaphysical subtleties of their schools, could not but discover in
light an element of the utmost importance in natural operations. The
alchemists regarded the luminous principle as a most subtile fluid,
capable of interpenetrating and mingling with gross matter: gold
being supposed to differ from the baser metals only in containing a
larger quantity of this ethereal essence.[85] Modern science, after
investigating most attentively a greater number of the phenomena of
light, has endeavoured to assist the inquiry by the aid of hypotheses.
Newton, in a theory, which exhibits the refined character of that
great philosopher’s mind, supposes luminous particles to dart from the
surfaces of bodies in all directions--that these infinitely minute
particles are influenced by the attracting and repelling forces of
matter, and thus turned back, or reflected, from their superficies in
some cases, and absorbed into their interstitial spaces in others.
Huyghens, on the contrary, supposes light to be caused by the waves or
vibrations of an infinitely elastic medium--ETHER--diffused through all
space, which waves are propagated in every direction from the luminous
body. In the first theory, a luminous particle is supposed actually to
come from the sun to the earth; in the other, the sun only occasions a
disturbance of the _ether_, which extends with great rapidity, in the
same manner as a wave spreads itself over the surface of a lake.
Nearly all the facts known in the time of Newton, and those discovered
by him, were explained most satisfactorily by his hypothesis; but
it was found they could be interpreted equally as the effects of
undulation, with the exception of the production of colour by prismatic
refraction. Although the labours of many gifted minds have been given,
with the utmost devotion, to the support of the vibratory theory,
this simple fact has never yet received any satisfactory explanation;
and there are numerous discoveries connected with the molecular and
chemical disturbances produced by the sun’s rays, which do not appear
to be explained by the hypothesis of emission or of undulation.
In both theories a wave motion is admitted, and every fact renders
it probable that this mode of progression applies not only to light,
but to the so-called _imponderable forces_ in general. Admitting,
therefore, the undulatory movement of luminous rays, we shall not stop
to consider those points of the discussion which have been so ably
dealt with by Young, Laplace, Fresnel, Biot, Fraunhofer, Herschel,
Brewster, and others, but proceed at once to consider the sources of
light, and its more remarkable phenomena.[86]
The sun is the greatest permanently luminous body we are acquainted
with, and that orb is continually pouring off light from its surface
in all directions at the rate, through the resisting medium of space
and of our own atmosphere, of 192,000 miles in a second of time. It has
been calculated, however, that light would move through a vacuum with
the speed of 192,500 miles in the same period. We, therefore, learn
that a ray of light requires eight minutes and thirteen seconds to
come from the sun to us. In travelling from the distant planet Uranus,
nearly three hours are exhausted; and from the nearest of the fixed
stars each ray of light requires more than six years to traverse the
intervening space between it and the earth. Allow the mind to advance
to the regions of nebulæ, and it will be found that hundreds of years
must glide away during the passage of their radiations. Consequently,
if one of those masses of matter, or even one of the remote fixed
stars, was “blotted out of heaven” to-day, several generations of
the finite inhabitants of this world would fade out of time before
the obliteration could be known to man. Here the immensity of space
assists us in our conception, limited though it be, of the for-ever of
eternity.[87]
All the planets of our system shine with reflected light, and the
moon, our satellite, also owes her silvery lustre to the sun’s
radiations. The fixed stars are, in all probability, suns shining from
the far distance of space, with their own self-emitted lights. By the
photometric researches of Dr. Wollaston, we learn, however, that it
would take 20,000 millions of such orbs as Sirius, the brightest of the
fixed stars, to afford as much light as we derive from the sun. The
same observer has proved that the brightest effulgence of the full moon
is yet 801,072 times less than the luminous power of our solar centre.
The cultivators of modern science are a bold race; not contented with
endeavouring to understand the physical earth, they are endeavouring
to comprehend the condition of the solar surface. The mind of man can
penetrate far into nature, and, as it were, feel out the mysteries of
untraversed space. The astronomer learns of a peculiar condition of
light, which is termed polarisation, and he learns by this, too, that
he can determine if from a bright luminous disc the light is derived
from a solid mass in a state of intense ignition, or from vapour in
an incandescent condition. He adds a polarising apparatus to his
telescopes, and he determines that the light we derive from the sun
is due to an envelope of vapour--burning, in all probability--only
with greater intensity, as the gas which we now employ. This
_Photosphere_--as it has been called by the late French philosopher
Arago, is found to be subjected to violent disturbances, and the dark
spots seen on the sun’s disc are now known to be openings through this
mysterious envelope of light, which enable us to look in upon the dark
body of the sun itself.
Luminous phenomena may be produced by various means--chemical action
is a source of light; and, under several circumstances in which the
laws of affinity are strongly exerted, a very intense luminous effect
is produced. Under this head all the phenomena of combustion are
included. In the electric spark we have the development of light;
and the arc which is formed between charcoal points at the poles of
a powerful voltaic battery affords us the most intense artificial
illumination with which we are acquainted. In addition to these, we
have the peculiar phenomena of phosphorescence arising from chemical,
calorific, electrical, actinic, and vital excitation, all of which must
be particularly examined.
From whatever source we procure light, it is the same in character,
differing only in intensity. In its action upon matter, we have the
phenomena of transmission, of reflection, of refraction, of colour, of
polarisation, and of vision, to engage our attention.
A beam of white light falls upon a plate of colourless glass, and it
passes freely through it, losing but little of its intensity; the
largest portion being lost by reflection from the first surface upon
which the light impinges. If the glass is roughened by grinding, we
lose more light by absorption and by reflection from the asperities of
the roughened surface; but if we cover that face with any oleaginous
fluid, as, for instance, turpentine, its transparency is restored. We
have thus direct proof that transparency to light is due to molecular
condition. This may be most strikingly shown by an interesting
experiment of Sir David Brewster’s:--
If a glass tube is filled with nitrous acid vapour, which is of a dull
red colour, it admits freely the passage of the red and orange rays
with some of the others, and, if held upright in the sunshine, casts
a red shadow on the ground; by gently warming it with a spirit-lamp,
whilst in this position, it acquires a much deeper and blacker colour,
and becomes almost impervious to any of the rays of light; but upon
cooling it again recovers its transparency.
It has also been stated by the same exact experimentalist, that having
brought a purple glass to a red heat, its transparency was improved, so
that it transmitted green, yellow, and red rays, which it previously
absorbed; but the glass recovered its absorptive powers as it cooled.
A piece of yellowish-green glass lost its transparency almost entirely
by being heated. Native yellow orpiment becomes blood-red upon being
warmed, when nearly all but the red rays are absorbed; and pure
phosphorus, which is of a pale yellow colour, and transmits freely all
the coloured rays upon being melted, becomes very dark, and transmits
no light.
Chemistry affords numerous examples of a very slight change of
condition, producing absolute opacity in fluids which were previously
diaphanous.[88]
Charcoal absorbs all the light which falls upon it, but in some
of its states of combination, and in the diamond, which is pure
carbon, it is highly transparent. Gold and silver beaten into thin
leaves are permeated by the green and blue rays, and the metals in
combination with acids are all of them more or less transparent. What
becomes of the light which falls upon and is absorbed by bodies,
is a question which we cannot yet, notwithstanding the extensive
observations that have been made by some of the most gifted of men,
answer satisfactorily. In all probability, as already stated, it
is permanently retained within their substances; and many of the
experiments of exciting light in bodies when in perfect darkness, by
the electric spark and other means, appear to support the idea of light
becoming latent or hidden.
No body is absolutely transparent; some light is lost in passing even
through ethereal space, and still more in traversing our atmosphere.
Amongst the most curious instances of absorption is that which is
uniformly discovered in the solar spectrum, particularly when we
examine it with a telescope. We then find that the coloured rays are
crossed by a great number of dark bands or lines, giving no light;
these are generally called Fraunhofer’s dark lines, as it was to the
indefatigable exertions of that experimentalist, and by the aid of his
beautiful instruments, that most of them were discovered and measured,
and enumerated, although they were previously noticed by Dr. Wollaston.
It is quite clear that those lines represent rays which have been
absorbed in their passage from the sun to the earth: although some
of them have no doubt undergone absorption within the limits of the
earth’s atmosphere, we have every reason to believe, with Sir John
Herschel, that the principal absorption takes place in the atmosphere
of the sun.[89]
It has been proved by Dr. Miller, that the number of those dark
lines is continually varying with the alteration of atmospheric
conditions;[90] and the evidences which have been afforded, of peculiar
states of absorption by the gaseous envelope of the earth,--during the
prosecution of investigations on the chemical agencies of the sun’s
rays,--are of a sufficiently convincing character.
It has been calculated by Bouguer, that if our atmosphere, in its
purest state, could be extended rather more than 700 miles from the
earth’s surface instead of nearly 40, as it is at present, the sun’s
rays could not penetrate it, and this globe would roll on in darkness
and silence, without a vestige of vegetable form or of animal life. In
the Hebrew version of the Mosaic History, the reading is, “Let light
appear:” may not this really mean that the earth’s atmosphere was so
cleared of obstructing vapours, that the solar rays were enabled to
reach the earth? The same calculation supposes that sea-water loses
all its transparency at the depth of 730 feet; but a dim twilight must
prevail much deeper in the ocean.
The researches of Professor Edward Forbes have proved, that at the
depth of 230 fathoms in the Ægean sea, the few shelled animals that
exist are colourless: no plants are found within that zone; and that
industrious naturalist fixes the zero of animal life of those waters at
about 300 fathoms.[91] Since these zones mark the rapidly diminishing
light, it is evident that where life ceases to be must be beyond the
limits to which life can penetrate.
Our atmosphere, charged with aqueous vapour, serves to shield us from
the intense action of the solar powers. By it we are protected from
the destructive influences of the sun’s light and heat; enjoy those
modified conditions which are most conducive to the healthful being of
organic forms; to it we owe “the blue sky bending over all,” and those
beauties of morning and evening twilight of which
---- Sound and motion own the potent sway.
Responding to the charm with its own mystery.
To defective transparency, or rather to the different degrees of it, we
must attribute, in part, the colours of permeable media. Thus, a glass
or fluid appears yellow to the eye, because it has the property of
admitting the permeation of a larger quantity of the yellow rays than
of any others;--red, because the red rays pass it with the greatest
freedom; and so on for every other colour. In most cases the powers of
transmission and of reflection are similar; but it is not so in all; a
variety of fluor spar, which, while it transmits green light, reflects
blue, and the precious opal, are striking instances to the contrary.
Some glasses, which transmit yellow light have the singular power of
dispersing blue rays from one surface; and a solution of quinine in
water acidulated with sulphuric acid, although perfectly transparent
and colourless when held between the eye and the light, exhibits, if
viewed in a particular direction, a lively cerulean tint. These effects
being supposed to be due to the conditions of the surface, have been
called _epipolic_ phenomena.[92]
The careful investigation of these phenomena has made us acquainted
with some very interesting facts, and indeed discovered to us a set
of luminous rays which were previously unknown. The dispersion of
blue light from the surface of some yellow glasses--such as have been
coloured by the oxide of silver--is of a different order from that
which takes place with the solution of sulphate of quinine, or with
the fluor spar. The first depends upon a peculiar condition of the
surface, while the latter phenomena are due to a dispersion which
takes place _within_ the solid or fluid. In addition to the sulphate
of quinine, and the fluor spar, we obtain the same results in a very
marked manner by a canary yellow glass, coloured with the oxide of
uranium, and by a decoction of the inner bark of the horse-chesnut
tree. Mr. Stokes, who has investigated this class of phenomena, and
proposes to call it _Fluorescence_, from its being naturally seen in
fluor-spar, has shown that the peculiar internal dispersion, and the
consequent alteration of the colour of the ray, is due to an alteration
in its refrangibility. Whether this hypothesis prove to be the correct
one or not, it is certain that there exists a set of rays of far higher
refrangibility than those seen in the ordinary Newtonian spectrum. This
may be shown in the following manner: taking either of the solutions
named, or a block of uranium glass, throw upon one face, by means of
a prism, a very pure spectrum. On looking _into_ the glass or fluid
there will be seen, commencing amidst the most refrangible rays, a
new set of spectral rays, struggling to make their way through the
absorbent medium. These are of a blue colour in the quinine or chesnut
solution, and green in the uranium glass, and are seen extending
themselves far beyond the most refrangible rays of the ordinary
Newtonian spectrum. This is the space over which those rays which
have the power of producing chemical changes, such as are rendered
familiar by the practice of Photography, are detected in their greatest
activity. It has, therefore, been supposed that these fluorescent rays
are the chemical rays rendered luminous by the alteration of their
refrangibility. This view has received much support from the fact that
the extra spectral rays are crossed with numerous dark lines, and that
in the chemical impressions these lines are marked by unchanged spaces
which exactly coincide with them. There is, however, much doubt of the
correctness of this, since, in the uranium glass of such a thickness
that these visible rays are quite absorbed, the chemical rays still
pass.
However, the whole question requires, and is receiving, the most
searching investigation. The discovery of these phenomena, which are
included under the term of Fluorescence, is of that interesting and
important character, that it must be ranked as the most decided advance
which has been made in physical optics since the days of Newton.
It is not improbable that those rays of such high refrangibility may,
although they are under ordinary circumstances invisible to the human
eye, be adapted to produce the necessary degree of excitement upon
which vision depends in the optic nerves of the night-roaming animals.
The bat, the owl, and the cat, may see in the gloom of night by the aid
of rays which are invisible to, or inactive on the eyes of man, or of
those animals which require the light of day for perfect vision.
It is a general law of the radiant forces, that whenever they fall upon
any surface, a portion is thrown back or reflected at the same time
as other portions are absorbed or transmitted. Upon this peculiarity
appear to depend the phenomena of natural colour in bodies.
The white light of the sun is well known to be composed of several
coloured rays. Or rather, according to the theory of undulations, when
the rate at which a ray vibrates is altered, a different sensation
is produced upon the optic nerve. The analytical examination of
this question shows, that to produce a red colour the ray of light
must give 37,640 undulations in an inch, and 458,000000,000000 in
a second. Yellow light requires 44,000 undulations in an inch, and
535,000000,000000 in a second; whilst the effect of blue results from
51,110 undulations within an inch, and 622,000000,000000 of waves in
a second of time.[93] The determination of such points as these is
among the highest refinements of science, and, when contrasted with the
most sublime efforts of the imagination, they must appear immeasurably
superior.
If a body sends back white light unchanged, it appears white; if the
surface has the property of altering the vibration to that degree which
is calculated to produce redness, the result is a red colour: the
annihilation of the undulations produces blackness. By the other view,
or the corpuscular hypothesis, the beam of white light is supposed to
consist of certain coloured rays, each of which has physical properties
peculiar to itself, and thus is capable of producing different
physiological effects. These rays falling upon a transparent or an
opaque body suffer more or less absorption, and being thus dissevered,
we have the effect of colour. A red body absorbs all the rays but the
red; a blue surface, all but the blue; a yellow, all but the yellow;
and a black surface absorbs the whole of the light which falls upon it.
That natural colours are the result of white light, and not innate
properties of the bodies themselves, is most conclusively shown by
placing coloured bodies in monochromatic light of another kind, when
they will appear either of the colour of that light, or, by absorbing
it, become black; whereas, when placed in light of their own character,
the intensity of colour is greatly increasing.
Every surface has, therefore, a peculiar constitution, by which it
gives rise to the diversified hues of nature. The rich and lively
green, which so abundantly overspreads the surface of the earth, the
varied colours of the flowers, and the numberless tints of animals,
together with all those of the productions of the mineral kingdom,
and of the artificial combinations of chemical manufacture, result
from powers by which the relations of matter to light are rendered
permanent, until its physical conditions undergo some change.
There is a remarkable correspondence between the geographical position
of a region and the colours of its plants and animals. Within the
tropics, where
“The sun shines for ever unchangeably bright,”
the darkest green prevails over the leaves of plants; the flowers
and fruits are tinctured with colours of the deepest dye, whilst the
plumage of the birds is of the most variegated description and of the
richest hues. In the people also of these climes there is manifested
a desire for the most striking colours, and their dresses have all
a distinguishing character, not of shape merely, but of chromatic
arrangement. In the temperate climates everything is of a more subdued
variety: the flowers are less bright of hue; the prevailing tint of the
winged tribes is a russet brown; and the dresses of the inhabitants of
these regions are of a sombre character. In the colder portions of the
earth there is but little colour; the flowers are generally white or
yellow, and the animals exhibit no other contrast than that which white
and black afford. A chromatic scale might be formed, its maximum point
being at the equator, and its minimum at the poles.[94]
The influence of light on the colours of organized creation is well
shown in the sea. Near the shores we find sea-weeds of the most
beautiful hues, particularly on the rocks which are left dry by the
tides; and the rich tints of the actiniæ, which inhabit shallow water,
must have been often observed. The fishes which swim near the surface
are also distinguished by the variety of their colours, whereas those
which live at greater depths are grey, brown, or black. It has been
found that after a certain depth, where the quantity of light is so
reduced that a mere twilight prevails, the inhabitants of the ocean
become nearly colourless. That the sun’s ray alone gives to plants the
property of reflecting colour is proved by the process of blanching, or
_etiolation_, produced by artificially excluding the light.
By a triangular piece of glass--a prism,--we are enabled to resolve
light into its ultimate rays. The white pencil of light which falls
on the first surface of the prism is bent from its path, and coloured
bands of different colours are obtained. These bands or rays observe a
curious constancy in their positions: the red ray is always the least
bent out of the straight path: the yellow class comes next in the
order of refrangibility; and the blue are the most diverted from the
vertex of the prism. The largest amount of illuminating power exists
in the yellow ray, and it diminishes towards either end.[95] It is not
uninteresting to observe something like the same variety of colour
occurring at each end of the prismatic spectrum. The strict order
in which the pure and mixed coloured rays present themselves is as
follows:--
1. The _extreme red_: a ray which can only be discovered when the eye
is protected from the glare of the other rays by a cobalt blue glass,
is of a crimson character--a mixture of the _red_ and the _blue_, red
predominating.[96]
2. The _red_: the first ray visible under ordinary circumstances.
3. The _orange_: red passing into and combining with yellow.
4. The _yellow_: the most intensely luminous of the rays.
5. The _green_: the yellow passing into and blending with the blue.
6. The _blue_: in which the light very rapidly diminishes.
7. The _indigo_: the dark intensity of blue.
8. The _violet_: the _blue_ mingled again with the _red_--blue being in
excess.
9. The _lavender grey_: a neutral tint, produced by the combination of
the red, blue, and yellow rays, which is discovered most easily when
the spectrum is thrown upon a sheet of turmeric paper.
10. The fluorescent rays: which are either a _pure silvery blue_ or a
delicate _green_.
Newton regarded the spectrum as consisting of seven colours of
definite and unvarying refrangibility. Brewster and others appear to
have detected a great diffusion of the colours over the spectrum,
and regard white light as consisting only of three rays, which in the
prismatic images overlap each other; and from these--red, yellow,
and blue--all the others can be formed by combination in varying
proportions. The truth will probably be found to be, that the ordinary
prismatic spectrum is a compound of two spectra:--that is, as we have
the ordinary rainbow, and a supplementary bow, the colours of which are
inverted, so the extraordinary may be somewhat masked by the intense
light of the ordinary spectrum; and yet by overlapping produce the
variations of colour in the rays. We have already examined the heating
power found in these coloured bands, which, although shown to be in a
remarkable manner in constant agreement with the colour of a particular
ray, is not directly connected with it; that is, not as the effect of a
cause, or the contrary. The chemical action of the solar rays, to which
from its important bearings we shall devote a separate chapter, has, in
like manner with heat, been confounded with the sun’s luminous power;
but although associated with light and heat, and modified by their
presence, it must be distinguished from them.
We find the maximum of heat at one end of the spectrum, and that of
chemical excitation at the other--luminous power observing a mean point
between them. Without doubt we have these powers acting reciprocally,
modifying all the phenomena of each other, and thus giving rise to the
difficulties which beset the inquirer on every side.
We have beautiful natural illustrations of luminous refraction in
the rainbow and in the halo: in both cases the rays of light being
separated by the refractive power of the falling rain drop, or the
vesicles which form the moisture constituting a fog. In the simple toy
of the child--the soap-bubble floating upon the air--the philosopher
finds subjects for his contemplation; and from the unrivalled play of
colours which he discovers in that attenuated film, he learns that the
varying thicknesses of surfaces influence, in a most remarkable manner,
the colours of the sunbeam. Films of oil floating upon water present
similar appearances; and the colours developed in tempering steel
are due entirely to the thickness of the oxidized surface produced
by heat. There have lately been introduced some beautiful specimens
of paper rendered richly iridescent by the following process:--A
solution of a gum resin in chloroform is floated upon water, where
it forms a film giving all the colours of Newton’s rings. A sheet of
paper which has been previously sunk in the water is carefully lifted,
and the film thus removed adheres with great firmness to the paper,
and produces this rich and curious play of colour. The rich tints
upon mother-of-pearl, in the feathers of many birds, the rings seen
in the cracks of rock-crystal, or between the unequal faces of two
pieces of glass, and produced by many chemical and indeed mechanical
operations--are all owing to the same cause;--the refraction of the
luminous pencil by the condition of the film or surface. If we take
one of those steel ornaments which are formed by being covered with
an immense number of fine lines, it will be evident that these striæ
present many different angles of reflection, and that, consequently,
the rays thrown back will, at some point or another, have a tendency
to cross each other. The result of this is, that the quantity of
light is augmented at some points of intersection, and annihilated at
others.[97] Out of the investigation of the phenomena of diffraction,
of the effects of thin and thick plates upon light, and the results of
interference, has arisen the discovery of one of the most remarkable
conditions within the range of physical science.
_Two bright lights may be made to produce darkness._--If two pencils
of light radiate from two spots very close to each other in such a
manner that they cross each other at a given point, any object placed
at that line of interference will be illuminated with the sum of the
two luminous pencils. If we suppose those rays to move in waves, and
the elevation of the wave to represent the maximum of luminous effect,
then the two waves meeting, when they are both at the height of their
undulation, will necessarily produce a spot of greater intensity. If
now we so arrange the points of radiation, that the systems of luminous
waves proceed irregularly, and that one arrives at the screen half an
undulation before the other, the one in elevation falling into the
depression of the other, a mutual annihilation is the consequence. This
fact, paradoxical as it may appear, was broadly stated by Grimaldi,
in the description of his experiments on the inflection of light, and
has been observed by many others. The vibratory hypothesis, seizing
upon the analogy presented by two systems of waves in water, explains
this plausibly, and many similar phenomena of what is called the
_interference of light_; but still upon examination it does not appear
that the explanation is quite free from objection.[98]
Another theory, not altogether new to us, it being indicated in Mayer’s
hypothesis of three primary colours (1775), and to be found as a
problem in some of the Encyclopædias of the last century, has been put
forth, in a very original manner, by that master-mind of intellectual
Germany, Goethe; and from the very comprehensive views which this
poet-philosopher has taken of both animal and vegetable physiology
(views which have been adopted by some of the first naturalists of
Europe), we are bound to receive his theory of colours with every
respect and attention.
Goethe regards colour as the “thinning” of light; for example, by
obstructing a portion of white light, yellow is produced; by reducing
it still farther, red is supposed to result; and by yet farther
retarding the free passage of the beam, we procure a blue colour, which
is the next remove from blackness, or the absence of light. There is
truth in this; it bears about it a simplicity which will satisfy many
minds; by it many of the phenomena of colour may be explained: but it
is insufficient for any interpretation of several of those laws to
which the other theories do give us some insight.
Newton may have allowed himself to be misled by the analogy presented
between the seven rays of the spectrum and the notes in an octave.
The mystic number, seven, may have clung like a fibre of the web of
superstition to the cloak of the great philosopher; but the attack made
by Goethe upon the Newtonian philosophy betrays the melancholy fact of
his being diseased with the lamentable weakness of too many exalted
minds--an overweening self-esteem.
The polarization of light, as it has been unfortunately
called--unfortunately, as conveying an idea of determinate and
different points or poles, which only exists in hypothetical
analogy--presents to us a class of phenomena which promise to unclose
the mysterious doors of the molecular constitution of bodies.
This remarkable condition, as produced by the reflection of light from
glass at a particular angle, was first observed by Malus, in 1808,[99]
when amusing himself by looking at the beams of the setting sun,
reflected from the windows of the Luxembourg Palace through a double
refracting prism. He observed that when the prism was in one position,
the windows with their golden rays were visible; but that turned
round a quarter of a circle from that position, the reflected rays
disappeared although the windows were still seen.
The phenomenon of double refraction was noticed, in the first instance,
by Erasmus Bartholin, in Iceland-spar, a crystal the primary form of
which is a rhombohedron; who perceived that the two images produced by
this body were not in the same physical conditions.[100] It was also
studied by Huyghens and Sir Isaac Newton, and to our countryman we owe
the singular idea that a ray of light emerging from such a crystal
has _sides_. This breaking up of the beam of light into two,--which
is shown by looking through a pin-hole on a card through a crystal of
Iceland spar, when two holes become visible, is due to the different
states of tension in which the different layers constituting the
crystal exist.
In thus separating the ray of light into two rays, the condition
called polarisation has been produced, and by experiment we discover
that the single ray has properties different from those of the compound
or ordinary ray.
It is somewhat difficult to explain what is meant by, and what are the
conditions of, _polarised light_. In the first instance let us see by
what methods this peculiar state may be brought about.
If we reflect a ray of light from the surface of any body, fluid or
solid, but not metallic, at an angle between 53° and 68° it undergoes
what has been called _plane polarisation_. It may also be produced by
the refraction of light from several refracting surfaces acting upon
the pencil of light in succession; as by a bundle of plates of glass.
Each surface polarises a portion of the pencil, and the number of
plates necessary to polarise a whole beam depends upon the intensity of
the beam and the angle of incidence. Thus, the light of a wax candle
is wholly polarised by forty-seven plates of glass at an angle of 40°
41'; while at an angle of 79° 11' it is polarised by eight plates.
Again, plane polarisation may be produced by the double refraction of
crystals. Each of the two pencils is polarised, like light reflected
from glass at an angle of 56° 45', but in opposite planes.
Non-scientific readers will still ask,--What is this mysterious
condition of light which is produced by reflection and refraction at
peculiar angles to the incident ray. It is one of the most difficult of
problems to express in popular language. The conditions are, however,
these:--
An ordinary ray of light will be reflected from a reflecting surface at
whatever angle that surface may be placed in relation to the incident
beam.
A polarised ray of light is not reflected in all positions of the
reflecting surface.
An ordinary ray of light is freely transmitted through a transparent
medium, as glass, in whatever position it may be placed relative to the
source of light.
A polarised ray of light is not transmitted in all the positions of the
permeable medium.
Supposing a plate of glass is presented at the angle 56° to a polarised
ray, and the plane of incidence or reflexion is at right angles to the
plane of polarisation of the ray, _no light is reflected_. If we turn
the plate of glass round through 90°, when the plane of reflexion is
parallel to that of polarisation _the light is reflected_. If we turn
the plate round another 90°, so that the plane of reflexion and of
polarisation are parallel to each other, again _no light is reflected_;
and if we turn it through another 90° the reflection of the ray again
takes place.
Precisely the same result takes place when, instead of being reflected,
the polarised ray is transmitted.
Some substances have peculiar polarizing powers: _the tourmaline_ is a
familiar example. If a slice of tourmaline is taken, and we look at a
common pencil of light through it, we see it in whatever position we
may place the transparent medium. If, however, we look at a pencil of
polarised light, and turn the crystal round, it will be found that in
two positions the light is stopped, and that in two other positions it
passes freely through it to the eye.
By way of endeavouring to conceive something of what may be the
conditions which determine this very mysterious state, let us suppose
each ray of light to vibrate in two planes at right angles to each
other: one wave being vertical and the other horizontal. We have many
examples of this compound motion. The mast of a ship, by the force with
which she is urged through the water, describes a vertical wave, while
by the roll of the billows across which she sails, a lateral undulation
is produced at the same time. We may sometimes observe the same thing
when a field of corn is agitated by a shifting wind on a gusty day.
The hypothesis therefore is, that every ray of ordinary light consists
of two rays vibrating in different planes; and that these rays,
separated one from the other, have the physical conditions which we
call _polarized_.
The most transparent bodies may be regarded as being made up of atoms
arranged in certain planes. Suppose the plane of lamination of any
substance to be vertical in position, it would appear that the ray
which has a vertical motion passes it freely, whereas if we turn the
body round so that the planes of lamination are at right angles to the
plane of vibration of the ray, it cannot pass.
That some action similar to that which it is here endeavoured to
express in popular language does take place, is proved by the
correctness of the results deduced by rigid mathematical analyses
founded on this hypothesis.
There are two other conditions of the polarization of light--called
_circular_ and _elliptical_ polarization. The first is produced by
light when it is twice reflected from the second surface of bodies at
their angle of maximum polarization, and the second by reflexions from
the surfaces of metals at angles varying from 70° 45' to 78° 30'. The
motion of the wave in the first is supposed to be circular, or to be
that which is represented by looking along the centre of a corkscrew
as it is turned round. At every turn of the medium effecting _circular
polarization_ the colour of the ray of light is changed after a uniform
order. If turned in one direction, they change through red, orange,
yellow, green, and violet; and if in the other direction, the colours
appear in the contrary order.
The variety of striking effects produced by the polarization of light;
the unexpected results which have sprung from the investigation of the
laws by which it is regulated; and the singular beauty of many of its
phenomena, have made it one of the most attractive subjects of modern
science.
Ordinary light passes through transparent bodies without producing any
very striking effects in its passage; but this _extraordinary_ beam
of light has the power of insinuating itself between the molecules
of bodies, and by illuminating them, and giving them every variety
of prismatic hue, of enabling the eye to detect something of the
structure of the mass. The chromatic phenomena of polarized light are
so striking, that no description can convey an adequate idea of their
character.
Spectra more beautiful and intense than the prismatic image,--systems
of rings far excelling those of thin plates,--and forms of the most
symmetric order, are constantly presenting themselves, as the polarized
ray is passed through various transparent substances; the path of the
ray indicating whether the crystal has been formed round a single
nucleus or axis, or whether it has been produced by aggregation
around two axes. The coloured rings, and the dark or luminous crosses
which distinguish the path of the polarized ray, are respectively
due to different states of tension amongst the particles, although
those differences are so slight, that no other means is of sufficient
delicacy to detect the variation.
The poetry which surrounds these, in every way, mysterious conditions
of the solar beam, is such, that it is with difficulty that imagination
is restrained by the stern features of truth. The uses of this peculiar
property in great natural phenomena are not yet made known to us; but,
since we find on every side of us the natural conditions for thus
separating the beam of light, and effecting its polarization, there
must certainly be some most important end for which it is designed by
Him who said, “Let there be Light.”
It must not be forgotten that we have at command the means of showing
that the chromatic phenomena of polarized light are due to atomic
arrangement. By altering the molecular arrangement of transparent
bodies, either by heat or by mere mechanical pressure, the unequal
tension or strain of the particles is at once indicated by means of
the polarized ray of light and its rings of colour. Differences in the
chemical constitution of bodies, too slight to be discovered by any
other mode of analysis, can be most readily and certainly detected by
this luminous investigator of the molecular forces.[101]
Although we cannot enter into an examination of all the conditions
involved in the polarization of, and the action of matter on, ordinary
light, it will be readily conceived, from what has been already stated,
that some most important properties are indicated, beyond those which
science has made known.
Almost every substance in nature, in some definite position, appears
to have the power of producing this change upon the solar ray, as
may be satisfactorily shown by examining them with a polarizing
apparatus.[102] The sky at all times furnishes polarized light, which
is most intense where it is blue and unclouded, and the point of
maximum polarization is varied according to the relative position of
the sun and the observer. A knowledge of this fact has led to the
construction of a “Solar Clock,”[103] with which the hour can be
readily determined by examining the polarized condition of the sky.
It has been stated, that chemical change on the Daguerreotype plates
and on photographic papers is more readily produced by the polarized
than by the ordinary sunbeam.[104] If this fact be established by
future investigations, we advance a step towards the discovery so much
desiderated of the part it plays in natural operations.
The refined and accurate investigations of Dr. Faraday stand
prominently forward amid those which will redeem the present age from
the charge of being superficial, and they will, through all time, be
referred to as illustrious examples of the influence of a love of truth
for truth’s sake, in entire independence of the marketable value, which
it has been unfortunately too much the fashion to regard. The searching
examination made by this “interpreter of nature” into the phenomena
of electricity in all its forms, has led him onward to trace what
connexion, if any, existed between this great natural agent and the
luminous principle.
By employing that subtile analyzer, a polarized ray, Dr. Faraday
has been enabled to detect and exhibit effects of a most startling
character. He has proved magnetism to have the power of influencing a
ray of light in its passage through transparent bodies. A polarized ray
is passed through a piece of glass or a crystal, or along the length
of a tube filled with some transparent fluid, and the line of its path
carefully observed; if, when this is done, the solid or fluid body is
brought under powerful magnetic influence, such as we have at command
by making a very energetic voltaic current circulate around a bar of
soft iron, it will be found that the polarized light is disturbed;
that, indeed, it does not permeate the medium along the same line.[105]
This effect is most strikingly shown in bodies of the greatest density,
and diminished in fluids, the particles of which are easily moveable
over each other, and has not hitherto been observed in any gaseous
medium. The question, therefore, arises,--does magnetism act directly
upon the ray of light, or only indirectly, by producing a molecular
change in the body through which the ray is passing? This question,
so important in its bearings upon the connexion between the great
physical powers, will, no doubt, before long receive a satisfactory
reply. A medium is necessary to the production of the result, and, as
the density of the medium increases, the effect is enlarged: it would
therefore appear to be due to a disturbance by magnetic force of the
particles which constitute the medium employed.
Without any desire to generalize too hastily, we cannot but express
a feeling,--amounting to a certainty in our own mind,--that those
manifestations of luminous power, connected with the phenomena of
terrestrial magnetism, which are so evident in all the circumstances
attendant upon the exhibition of Aurora Borealis, and those luminous
clouds which are often seen, independent of the Northern Lights, that
a very intimate, relation exists between the solar radiations and that
power which so strangely gives polarity to this globe of ours.
In connexion with the mysterious subject of solar light, it is
important that we should occupy a brief space in these pages with the
phenomena of vision, which is so directly dependent upon luminous
radiation.
The human eye has been rightly called the “masterpiece of divine
mechanism;” its structure is complicated, yet all the adjustments of
its parts are as simple as they are perfect. The eye-ball consists of
four coats. The cornea is the transparent coat in front of the globe;
it is the first optical surface, and this is attached to the sclerotic
membrane, filling up the circular aperture in the white of the eye;
the choroid coat is a very delicate membrane, lining the sclerotic,
and covered with a perfectly black pigment on the inside; and close to
this lies the most delicately reticulated membrane, the retina, which
is, indeed, an extension of the optic nerve. These coats enclose three
humours,--the aqueous, the vitreous, and the crystalline humours.
The eye, in its more superficial mechanical arrangements, presents
exactly the same character as a camera obscura, the cornea and
crystalline lens receiving the images of objects refracting and
inverting them; but how infinitely more beautiful are all the
arrangements of the organ of vision than the dark chamber of Baptista
Porta![106] The humours of the eye are for the purpose of correcting
the aberrations of light, which are so evident in ordinary lenses,
and for giving to the whole an achromatic character. Both spherical
and chromatic aberration are corrected, the latter not entirely, and
by the agency of the cornea and the crystalline lens perfect images
are depicted on the retina, in a similar way to those very charming
pictures which present themselves in the table of the camera obscura.
The seat of vision has been generally supposed to be the retina;
but Mariotte has shown that the base of the optic nerve, which is
immediately connected with the retina, is incapable of conveying an
impression to the brain. The choroid coat, which lies immediately
behind the retina, is regarded by Mariotte and Bernoulli as the more
probable seat of vision. The retina, being transparent, offers no
obstruction to the passage of the light onward to the black surface of
the choroid coat, from which the vibrations are, in all probability,
communicated to the retina and conveyed to the brain. Howbeit, upon one
or the other of these delicate coats a distinct image is impressed
by light, and the communication made with the brain possibly by a
vibratory action. We may trace up the phenomena of vision to this
point; we may conceive undulations of light, differing in velocity and
length of wave, occasioning corresponding tremors in the neuralgic
system of the eye; but how these vibrations are to communicate
correct impressions of length, breadth, and thickness, no one has yet
undertaken to explain.
It has, however, been justly said by Herschel:--
“It is the boast of science to have been able to trace so far the
refined contrivances of this most admirable organ, not its shame to
find something still concealed from scrutiny; for, however anatomists
may differ on points of structure, or physiologists dispute on modes
of action, there is that in what we _do_ understand of the formation
of the eye, so similar, and yet so infinitely superior to a product
of human ingenuity; such thought, such care, such refinement, such
advantage taken of the properties of natural agents used as mere
instruments for accomplishing a given end, as force upon us a
conviction of deliberate choice and premeditated design, more strongly,
perhaps, than any single contrivance to be found whether in art or
nature, and renders its study an object of the greatest interest.”[107]
Has the reader ever asked himself why it is, having two eyes, and
consequently two pictures produced upon the tablets of vision, that we
see only one object? According to the law of visible direction, all the
rays passing through the crystalline lenses converge to one point upon
the retina,--and as the two images are coincident and nearly identical,
they can only produce the sensation of one upon the brain.
When we look at any round object, as the ornamented moderator lamp
before us, first with one eye, and then with the other, we discover
that, with the right eye, we see most of the right-hand side of the
lamp, and with the left eye more of the left-hand side. These two
images are combined, and we see an object which we know to be round.
This is illustrated in a most interesting manner by the little optical
instrument, _the Stereoscope_. It consists either of two mirrors placed
each at an angle of 45°, or of two semi-lenses turned with their curved
sides towards each other. To view its phenomena, two pictures are
obtained by the camera obscura on photographic paper of any object in
two positions, corresponding with the conditions of viewing it with the
two eyes. By the mirrors or the lenses these dissimilar pictures are
combined within the eye, and the vision of an actually solid object is
produced from the pictures represented on a plane surface. Hence the
name of the instrument; which signifies, _Solid I see_.
Analogy is often of great value in indicating the direction in which to
seek for a truth; but analogical evidence, unless where the resemblance
is very striking, should be received with caution. Mankind are so ready
to leap to conclusions without the labour necessary for a faithful
elucidation of the truth, that too often a few points of resemblance
are seized upon, and an inference is drawn which is calculated to
mislead.
There is an idea that the phenomena of sound bear a relation to those
of light,--that there exists a resemblance between the chromatic
and the diatonic scales. Sound, we know, is conveyed by the beating
of material particles--the air--upon the auditory membrane of the
ear, which have been set in motion by some distant disturbance of
the medium through which it passes. Light has been supposed to act
on the optic nerve in the same manner. If we imagine colour to be
the result of vibrations of different velocities and lengths, we can
understand that under some of these tremors, first established on the
nerves, and through them conveyed to the brain, sensations of pain
or pleasure may result, in the same way as sharp or subdued sounds
are disagreeable or otherwise. Intensely coloured bodies do make an
impression upon perfectly blind men; and those who, being born blind,
know no condition of light or colour, will point out a difference
between strongly illuminated red and yellow media. When the eyes are
closed we are sensible to luminous influence, and even to differences
of colour. We must consequently infer that light produces some peculiar
action upon the system of nerves in general; this may or may not
be independent of the chemical agency of the solar radiations; but
certainly the excitement is not owing to any calorific influence. The
system of nerves in the eye is more delicately organized, and of course
peculiarly adapted to all the necessities of vision.
Thus far some analogy does appear to exist between light and sound;
but the phenomena of the one are so much more refined than those of
the other--the impressions being all of them of a far more complicated
character, that we must not be led too far by the analogical evidence
in referring light, like sound, to mere material motion.
It was a beautiful idea that real impressions of external objects
are made upon the seat of vision, and that they are viewed, as in a
picture, by something behind the screen,--that these pictures become
dormant, but are capable of being revived by the operations of the mind
in peculiar conditions; but we can only regard it as a philosophical
speculation of a poetic character, the truth or falsehood of which we
are never likely to be enabled to establish.[108]
That which sees will never itself be visible. The secret principle
of sensation,--the mystery of the life that is in us,--will never be
unfolded to finite minds.
Numerous experiments have been made from time to time on the influence
of light upon animal life. It has been proved that the excitement of
the solar rays is too great for the healthful growth of young animals;
but, at the same time, it appears probable that the development of
the functional organs of animals requires, in some way, the influence
of the solar rays. This might, indeed, have been inferred from the
discovery that animal life ceases in situations from which light is
absolutely excluded. The instance of the Proteus of the Illyrian lakes
may appear against this conclusion. This remarkable creature is found
in the deep and dark recesses of the calcareous rocks of Adelsburg, at
Sittich; and it is stated, also in Sicily, and in the Mammoth caves
of Kentucky. Sir Humphry Davy describes the Proteus anguinus as “an
animal to whom the presence of light is not essential, and who can
live indifferently in air and in water, on the surface of the rock,
or in the depths of the mud.” The geological character of rocks,
however, renders it extremely probable that these animals may have
descended with the water, percolating through fissures from very near
the surface of the ground. All the facts with which science has made us
acquainted--and both natural and physical science has been labouring
with most untiring industry in the pursuit of truth--go to prove
that light is absolutely necessary to organization. It is possible
the influence of the solar radiations may extend beyond the powers
of the human senses to detect luminous or thermic action, and that
consequently a development of animal and vegetable forms may occur
where the human eye can detect no light; and under such conditions
the Proteus may be produced in its cavernous abodes, and also those
creatures which live buried deep in mud. Some further consideration of
the probable agency of light will occupy us, when we come to examine
the phenomena of vital forces.
Light is essentially necessary to vegetable life; and to it science
refers the powers which the plant possesses of separating carbon from
the air breathed by the leaves, and secreting it within its tissues for
the purpose of adding to its woody structure. As, however, we have, in
the growing plant, the action of several physical powers exerted to
different ends at the same time, the remarkable facts which connect
themselves with vegetable chemistry and physiology are deferred for a
separate examination.
The power of the solar rays to produce in bodies that peculiar gleaming
light which we call phosphorescence, and the curious conditions under
which this phenomenon is sometimes apparent, independent of the sun’s
direct influence, present a very remarkable chapter in the science of
luminous powers.
The phosphorescence of animals is amongst the most surprising of
nature’s phenomena, and it is not the less so from our almost entire
ignorance of the cause of it. Many very poetical fancies have been
applied in description of these luminous creations; and imagination
has found reason why they should be gifted with these extraordinary
powers. The glow-worm lights her lamp to lure her lover to her bower,
and the luminous animalcules of the ocean are employed in lighting up
the fathomless depths where the sun’s rays cannot penetrate, to aid its
monsters in their search for prey. “The lamp of love--the pharos--the
telegraph of the night,--which scintillates and marks, in the silence
of darkness, the spot appointed for the lover’s rendezvous,”[109] is
but a pretty fiction; for the glow-worm shines in its infant state,
in that of the larva, and when in its aurelian condition. Of the
dark depths of the ocean it may be safely affirmed that no organized
creation lives or moves in its grave-like silence to require this
fairy aid. Fiction has frequently borrowed her creations from science.
In these cases science appears to have made free with the rights of
fiction.
The glow-worms (_lampyris noctiluca_), it is well known, have the
power of emitting from their bodies a beautiful pale bluish-white
light, shining during the hours of night in the hedge-row, like crystal
spheres. It appears, from the observations of naturalists, that these
insects never exhibit their light without some motion of the body or
legs;--from this it would seem that the phosphorescence was dependent
upon nervous action, regulated at pleasure by the insect; for they
certainly have the power of obscuring it entirely. If the glow-worm is
crushed, and the hands or face are rubbed with it, luminous streaks,
similar to those produced by phosphorus, appear. They shine with
greatly increased brilliancy in oxygen gas and in nitrous oxide. From
these facts may we not infer that the process by which this luminosity
is produced, whatever it may be, has a strong resemblance to that of
respiration?
There are several varieties of flies, and three species of beetles of
the genus _Elater_, which have the power of emitting luminous rays.
The great lantern-fly of South America is one of the most brilliant, a
single insect giving sufficient light to enable a person to read. In
Surinam a very numerous class of these insects are found, which often
illuminate the air in a remarkable manner. In some of the bogs of
Ireland a worm exists which gives out a bright green light; and there
are many other kinds of creatures which, under certain circumstances,
become luminous in the dark. This is always dependent upon vitality;
for all these animals, when deprived of life, cease to shine.
At the same time we have many very curious instances of phosphorescence
in dead animal and vegetable matter; the lobster among the Crustacea,
and the whiting among fishes, are striking examples; decayed wood
also emits much light under certain conditions of the atmosphere.
This development of light does not appear to be at all dependent upon
putrefaction; indeed, as this process progresses, the luminosity
diminishes. We cannot but imagine that this light is owing, in the
first place, to direct absorption by, and fixation within, the
corpuscular structure of those bodies, and that it is developed by the
decomposition of the particles under the influence of our oxygenous
atmosphere.
The pale light emitted by phosphorus in the dark is well known; and
this is evidently only a species of slow combustion, a combination of
the phosphorus with the oxygen of the air. Where there is no oxygen,
phosphorus will not shine; its combustion in chlorine or iodine vapour
is a phenomenon of a totally different character from that which we are
now considering. This phosphorescence of animal and vegetable matter
has been regarded as something different from the slow combustion of
phosphorus; but, upon examination, all the chemical conditions are
found to be the same, and it is certainly due to a similar chemical
change.
The luminous matter of the dead whiting or the mackerel may be
separated by a solution of common salt or of sulphate of magnesia; by
concentrating these solutions the light disappears; but it is again
emitted when the fluid is diluted. The entire subject is, however,
involved in the mystery of ignorance, although it is a matter quite
within the scope of any industrious observer. The self-emitted light
of the carbuncle of the romancer is realized in these remarkable
phenomena.
The phosphorescence of some plants and flowers is not, perhaps, of
the same order as that which belongs to either of the conditions we
have been considering. It appears to be due rather to an absorption of
light and its subsequent liberation. If a nasturtium is plucked during
sunshine, and carried into a dark room, the eye, after it has reposed
for a short time, will discover the flower by a light emitted from its
leaves.
The following remarkable example, and an explanation of it by the poet
Goethe, is instructive:--
“On the 19th of June, 1799, late in the evening, when the twilight was
deepening into a clear night, as I was walking up and down the garden
with a friend, we very distinctly observed a flame-like appearance
near the oriental poppy, the flowers of which are remarkable for their
powerful red colour. We approached the place, and looked attentively
at the flowers, but could perceive nothing further, till at last, by
passing and repassing repeatedly, while we looked side-ways on them, we
succeeded in renewing the appearance as often as we pleased. It proved
to be a physiological phenomenon, and the apparent corruscation was
nothing but the spectrum of the flower in the complementary blue-green
colour. The twilight accounts for the eye being in a perfect state
of repose, and thus very susceptible, and the colour of the poppy is
sufficiently powerful in the summer twilight of the longest days to act
with full effect, and produce a complementary image.”[110]
The leaves of the _œnothera macrocarpa_ are said to exhibit phosphoric
light when the air is highly charged with electricity. The agarics
of the olive-grounds of Montpelier have been observed to be luminous
at night; but they are said to exhibit no light, even in darkness,
_during the day_. The subterranean passages of the coal mines near
Dresden are illuminated by the phosphorescent light of the _rhizomorpha
phosphoreus_, a peculiar fungus. On the leaves of the Pindoba palm, a
species of agaric grows which is exceedingly luminous at night; and
many varieties of the lichens, creeping along the roofs of caverns,
lend to them an air of enchantment by the soft and clear light which
they diffuse. In a small cave near Penryn, a luminous moss is abundant;
and it is also found in the mines of Hesse. According to Heinzmann, the
_rhizomorpha subterranea_ and _aidulæ_ are also phosphorescent.
It is but lately that a plant which abounds in the jungles in the
Madura district of the East Indies was sent to this country, which,
although dead, was remarkably phosphorescent; and, when in the
living state, the light which it emitted was extraordinarily vivid,
illuminating the ground for some distance. Those remarkable effects
may be due, in some cases, to the separation of phosphuretted hydrogen
from decomposing matter, and, in others, to some peculiar electric
manifestation.
The phosphorescence of the sea, or that condition called by fishermen
_brimy_, when the surface, being struck by an oar, or the paddle-wheels
of a steamer, gives out large quantities of light, has been attributed
to the presence of myriads of minute insects which have the power
of emitting light when irritated. The night-shining nereis (_Nereis
noctiluca_) emits a light of great brilliancy, as do several kinds of
the mollusca. The nereides attach themselves to the scales of fishes,
and thus frequently render them exceedingly luminous. Some of the
crustaceæ possess the same remarkable property;--twelve different
species of _cancer_ were taken up by the naturalists of the Zaire
in the Gulf of Guinea.[111] The _cancer fulgens_, discovered by Sir
Joseph Banks, is enabled to illuminate its whole body, and emits
vivid flashes of light. Many of the medusæ also exhibit powerful
phosphorescence.[112] These noctilucous creatures are, many of them,
exceedingly minute, several thousands being found in a tea-cup of sea
water. They float near the surface in countless myriads, and when
disturbed they give out brilliant scintillations, often leaving a train
of light behind them.[113] By microscopic examination no other fact has
been elicited than that these minute beings contain a fluid which, when
squeezed out, leaves a line of light upon the surface of water. The
appearance of these creatures is almost invariably on the eve of some
change of weather, which would lead us to suppose that their luminous
phenomena must be connected with electrical excitation; and of this,
the investigations of Mr. C. Peach, of Fowey, communicated to the
British Association at Birmingham, furnish the most satisfactory proofs
we have as yet obtained.
Benvenuto Cellini gave a curious account of a carbuncle which shone
with great brilliancy in the dark.[114] The same thing has been stated
of the diamond; but it appears to be necessary to procure these
emissions of light, that the minerals should be first warmed near a
fire. From this it may be inferred that the luminous appearance is
of a similar character to that of fluor spar, and of numerous other
earthy minerals, which, when exposed to heat, phosphoresce with great
brilliancy. Phosphorescent glow can also be excited in similar bodies
by electricity, as was first pointed out by Father Beccaria, and
confirmed by Mr. Pearsall.[115] These effects, it must be remembered,
are distinct from the electric spark manifested upon breaking white
sugar in the dark, or scratching sulphuret of zinc.
In the instances adduced there is not necessarily any exposure to the
sunshine required. It is probable that two, if not three, distinct
phenomena are concerned in the cases above quoted, and that all of them
are distinct from animal phosphorescence, or the luminous appearance
of vegetables. They, however, certainly prove, either that light is
capable of becoming latent, or that it is only a condition of matter,
in which it may be made manifest by any disturbance of the molecular
forces. We have, in answer to this, very distinct evidence that
some bodies do derive this property from the solar rays. Canton’s
phosphorus, which is a sulphuret of calcium, will, having been exposed
to the sun, continue luminous for some time after it is carried into
the dark; as will also the Bolognian stone,--a sulphuret of barium.
This result appears to be due to a particular class of the solar rays;
for it has been found, if these sulphurets, spread smoothly on paper,
are exposed to the influence of the solar spectrum for some little
time, and then examined in the dark, that luminous spaces appear,
exactly corresponding with the most refrangible rays, or those which
excite chemical change; and one very remarkable fact must not be
forgotten--the dark rays of the spectrum beyond the violet produce a
lively phosphorescence, which is _extinguished_ by the action of the
rays of least refrangibility, or the heat rays--whilst artificial heat,
such as a warm iron, produces a very considerable elevation of the
phosphorescent effect.[116] It is not improbable, that the fluorescent
rays of Mr. Stokes may be materially concerned in producing the
phenomena of phosphorescence: experiments are, however, required to
prove this.
In these allied phenomena we have effects which are evidently dependent
upon several dissimilar causes. The phosphorescence of the living
animal is due, without doubt, to nervous excitation: that of the living
vegetable to solar luminous influence; and in the case of the mosses of
caverns, &c. to the chemical agency of the sun’s rays, which appears to
be capable of conduction. In the dead organic matter we have a purely
chemical action developing the light, and in the inorganic bodies we
have peculiar molecular constitution, by which an absorption of light
appears to take place.
The subject is one of the greatest difficulty; the torch of science
is too dim to enable us to see the causes at work in producing these
marvellous effects. The investigation leads, to a certain extent, to
the elucidation of many of the secrets of luminous action; and the
determination of the question, whether light is an emanation from the
sun, or only a subtile principle diffused through all matter, which is
excited by solar influence, is intimately connected with the inquiry.
It has been stated that matter is necessary to the development of
light; that no luminous effect would be produced if it were not for
the presence of matter. Of this we not only have no proof, but such
evidence as we have is against the position. There is no loss of light
in the most perfect vacuum we can produce by any artificial means,
which should be the case if matter was concerned in the phenomena of
light, as a cause.
Colour is certainly a property regulated by material bodies; or
rather, the presence of matter is necessary to the production of
colour. Chlorine gas is a pale yellow, and nitrous vapour a yellowish
red. These and one or two other vapours, which are near the point of
condensation into fluids, are the only coloured gaseous or vaporiform
bodies. The sky is blue, because the material particles of the
atmosphere reflect back the blue rays. But we have more practical
illustrations than this. The flame of hydrogen burning with oxygen
gives scarcely any light; allow it to impinge on lime, a portion of
which is carried off by the heat of the flame, and the most intense
artificial light with which we are acquainted is produced. Hydrogen gas
alone gives a flame in which nearly all but the blue rays are wanting:
place a brush of steel or asbestos in it, and many of the other rays
are at once produced. An argand lamp, and more particularly the lamp
in which camphine--a purified turpentine,--is burnt, gives a flame
which emits most of the rays found in sunlight. Spirit of wine mixed
with water, warmed and ignited, gives only yellow rays; add nitrate
of strontian and they become red; but nitrate of barytes being mixed
with the fluid, they are changed to green and yellow; salts of copper
afford fine blue rays, and common salt intense yellow ones. Many of
these coloured rays and others can be produced in great power by the
use of various solid bodies introduced into flame. This has not been
sufficiently pointed out by authors; but it is clear from experiments
that light requires the presence of matter to enable it to diffuse its
coloured glories. How is it that the oxygen and hydrogen flame gives so
little light, and with a solid body present, pours forth such a flood
of brilliancy?
The production of artificial light by electrical and chemical agencies
will necessarily find some consideration under their respective
heads. There are numerous phenomena which connect themselves with
luminous power, or appear to do so, which, in the present state of
our knowledge, cannot come immediately under our attention. We are
compelled to reserve our limited space for those branches of science
which we are enabled to connect with the great natural operations
constantly going on around us. Many of these more abstruse results
will, however, receive some incidental notice when we come to examine
the operation of the combined physical forces on matter.
We see in light a principle which, if it has not its source in the
sun, is certainly dependent upon that luminary for its manifestations
and powers. From that “fountain of light” we find this principle
travelling to us at a speed which almost approaches the quickness of
thought itself; yet by the refinements of science we have been enabled
to measure its velocity with the utmost accuracy. The immortal poet of
our own land and language, in his creations of Ariel, that “tricksy
spirit,” who could creep like music upon the waters, and of the
fantastic Puck, who could girdle the earth in thirty minutes, appears
to have approached to the highest point to which mere imagination could
carry the human mind as to the powers of things ethereal. Science has,
since then, shown to man that this “spirit, fine spirit,” was a laggard
in his tasks, and a gross piece of matter, when compared with the
subtile essences which man, like a nobler Prospero, has now subdued to
do him service.
Light is necessary to life; the world was a dead chaos before its
creation, and mute disorder would again be the consequence of its
annihilation. Every charm which spreads itself over this rolling globe
is directly dependent upon luminous power. Colours, and probably,
forms, are the result of light; certainly the consequence of solar
radiations. We know much of the mysterious influences of this great
agent, but we know nothing of the principle itself. The solar beam has
been tortured through prismatic glasses and natural crystals; every
chemical agent has been tried upon it, every electrical force in the
most excited state brought to bear upon its operations, with a view
to the discovery of the most refined of earthly agencies; but it has
passed through every trial without revealing its secrets, and even the
effects which it produces in its path are unexplained problems, still
to tax the intellect of man.
Every animal and every plant alike proclaim that life and health are
due to light; and even the crystallizing forms of inorganic matter, by
bending towards it, confess its all-prevailing sway. From the sun to
each planet revolving around that orb, and to the remotest stars which
gleam through the vast immensity of heaven, we discover this power
still in its brightness, giving beauty and order to these unnumbered
creations; no less completely than to this small island of the universe
which we call our Earth. Through every form of matter we can mark
its power, and from all, we can, under certain conditions, evoke it
in lustre and activity. Over all and through all light spreads its
ethereal force, and manifests, in all its operations, powers which
might well exalt the mind of Plato to the idea of an omniscient and
omnipresent God. Science, with her Ithuriel wand, has, however, shown
that light is itself the effect of a yet more exalted cause, which we
cannot reach.
Indeed, the attentive study of the fine abstractions of science lifts
the mind from the grossness of matter, step by step, to the refinements
of immateriality, and there appear, shadowed out beyond the physical
forces which man can test and try, other powers still ascending, until
they reach the Source of every good and every perfect gift.
FOOTNOTES:
[84] “These--oxygen, hydrogen, nitrogen, and carbon--are the four
bodies, in fact, which, becoming animated at the fire of the sun,
the true torch of Prometheus, approve themselves upon the earth
the eternal agents of organisation, of sensation, of motion and of
thought.”--Dumas, _Leçons de Philosophie Chimique_, p. 100. Paris, 1837.
[85] It will be found in examining any of the works of the
alchemists,--particularly those of Geber, _De inveniendi arte Auri et
Argenti_, and his _De Alchemiâ_; Roger Bacon’s _Opus Majus, or Alchymia
Major_; Helvetius’ _Brief of the Golden Calf_; or Basil Valentine’s
_Currus Triumphalis_,--that in the processes of transmutation the solar
light was supposed to be marvellously effective. In Boyle’s _Sceptical
Chemist_ the same idea will be found pervading it.
Amid all their errors, the alchemists were assiduous workmen, and to
them we are indebted for numerous facts. Of them, and of their age, as
contrasted with our own, Gibbon remarks:--“Congenial to the avarice
of the human heart, it was studied in China, as in Europe, with equal
eagerness and equal success. The darkness of the middle ages ensured
a favourable reception to every tale of wonder; and the revival of
learning gave new vigour to hope, and suggested more specious arts
of deception. Philosophy, with the aid of experience, has at length
banished the study of alchemy; and the present age, however desirous of
riches, is content to seek them by the humbler means of commerce and
industry.”--_Decline and Fall_, vol. ii. p. 137.
[86] On the two theories the following maybe consulted:--Young,
_Supplement to Encyclopædia Britannica_, article _Chromatics_;
Fresnel, _Supplément à la Traduction Française de la 5ième édition du
Traité de Chimie de Thomson_, par Riffault, Paris, 1822; Herschel’s
Article, _Light_, in the Encyclopædia Metropolitana, and the French
Translation of it by Quetelet and Verhulst; Airy’s _Tract on the
Undulatory Theory_, in his Tracts, 2nd edition, Cambridge, 1831; Powel,
_The Undulatory Theory applied to Dispersion_, &c. p. 184; Lloyd’s
_Lectures_, Dublin, 1836-41; Cauchy, _Sur le Mouvement des Corps
élastiques_, Mémoires de l’Institut, 1827, vol. ix. p. 114; _Théorie de
la Lumière_, Ibid. vol. x. p. 293; M’Cullagh, _On Double Refraction_,
Ibid., vol. xvi.; _Geometrical Propositions applied to the Wave Theory
of Light_, Ibid., vol. xvii.; Sir David Brewster’s papers in the
Transactions of the Royal Society of Edinburgh, and the Philosophical
Magazine.
[87] _Results of Astronomical Observations made during the years
1834-38, at the Cape of Good Hope, &c._ By Sir John Herschel, Bart.,
K.H., D.C.L., F.R.S.--“In the contemplation of the infinite, in number
and in magnitude, the mind ever fails us. We stand appalled before this
mighty spectre of boundless space, and faltering reason sinks under the
load of its bursting conceptions. But, placed as we are on the great
locomotive of our system, destined surely to complete at least one
round of its ethereal course, and learning that we can make no apparent
advance on our sidereal journey, we pant with new ardour for that
distant bourne which we constantly approach without the possibility
of reaching it. In feeling this disappointment, and patiently bearing
it, let us endeavour to realise the great truth from which it flows.
It cannot occupy our mind without exalting and improving it.”--_Sir D.
Brewster_: North British Review.
[88] For examples of this, consult Graham’s _Elements of Chemistry_;
Brande’s _Manual of Chemistry_; or, indeed, any work treating of the
science. The formation of ink, by mixing two colourless solutions,
one of gallic acid and another of sulphate of iron, may be taken as a
familiar instance.
[89] Sir John Herschel, in his paper _On the Chemical Action of the
Rays of the Solar Spectrum on Preparations of Silver_, remarks that,
“it may seem too hazardous to look for the cause of this very singular
phenomenon in a real difference between the chemical agencies of those
rays which issue from the central portion of the sun’s disc, and those
which, emanating from its borders, have undergone the absorptive
action of a much greater depth of its atmosphere; and yet I confess
myself somewhat at a loss what other cause to assign for it. It must
suffice, however, to have thrown out the hint; remarking only, that
I have other, and, I am disposed to think, decisive evidence (which
will find its place elsewhere) of the existence of an absorptive solar
atmosphere, extending beyond the luminous one. The breadth of the
border, I should observe, is small, not exceeding 0·5 or 1/7 part of
the sun’s radius, and this, from the circumstances of the experiment,
must necessarily err in excess.”--Philosophical Transactions, 1840.
[90] _Experiments and Observations on some Cases of Lines in the
Prismatic Spectrum, produced by the passage of Light through Coloured
Vapours and Gases, and from certain Coloured Flames._ By W. A.
Miller, M.D., F.R.S., Professor of Chemistry in King’s College,
London.--Philosophical Magazine, vol. xxvii.
[91] _Report on the Mollusca and Radiata of the Ægean Sea, and on their
distribution, considered as bearing on Geology._ By Edward Forbes,
F.R.S., &c.--Reports of the British Association, vol. xii. Professor
Forbes remarks:--“A comparison of the testacea, and other animals of
the lowest zones, with those of the higher, exhibits a very great
distinction in the hues of the species, those of the depths being, for
the most part, white or colourless, while those of the higher regions,
in a great number of instances, exhibit brilliant combinations of
colour. The results of an enquiry into this subject are as follows:--
“The majority of shells of the lowest zone are white or transparent; if
tinted rose is the hue, a very few exhibit markings of another colour.
In the seventh region, white species are also very abundant, though
by no means forming a proportion so great as the eighth. Brownish
red, the prevalent hue of the brachiopoda, also gives a character
of colour to the fauna of this zone; the crustacea found in it are
red. In the sixth zone the colours become brighter, reds and yellows
prevailing,--generally, however, uniformly colouring the shell. In
the fifth region many species are banded or clouded with various
combinations of colours, and the number of white species has greatly
diminished. In the fourth, purple hues are frequent, and contrasts of
colour common. In the second and third, green and blue tints are met
with, sometimes very vivid; but the gayest combinations of colour are
seen in the littoral zone, as well as the most brilliant whites.
“The animals of Testacea, and the Radiata of the higher zones, are
much more brilliantly coloured than those of the lower, where they are
usually white, whatever the hue of the shell may be. Thus the genus
_Trochus_ is an example of a group of forms mostly presenting the
most brilliant hues both of shell and animal; but whilst the animals
of such species as inhabit the littoral zone are gaily chequered with
many vivid hues, those of the greater depth, though their shells are
almost as brightly covered as the coverings of their allies nearer the
surface, have their animals, for the most part, of a uniform yellow or
reddish hue, or else entirely white. The chief cause of this increase
of intensity of colour as we ascend is, doubtless, the increased amount
of light above a certain depth.”--p. 172.
[92] Ἁμὁρφωτα. _On the Epipolic Dispersion of Light_, being a paper
entitled, _On a case of Superficial Colour presented, by a homogeneous
liquid internally colourless_. By Sir J. F. W. Herschel, Bart, K.H.,
F.R.S., &c.--_An epipolized beam of light_ (meaning thereby a beam
which has once been transmitted through a quiniferous solution, and
undergone its dispersing action) _is incapable of further undergoing
epipolic dispersion_. In proof of this the following experiment may be
adduced,--
A glass jar being filled with a quiniferous solution, a piece of plate
glass was immersed in it vertically, so as to be entirely covered, and
to present one face directly to the incident light. In this situation,
when viewed by an eye almost perpendicularly over it, so as to graze
either surface very obliquely, neither the anterior nor posterior face
showed the slightest trace of epipolic colour. Now, the light, at its
egress from the immersed glass, entered the liquid under precisely the
same circumstances as that which, when traversing the anterior surface
of the glass jar, underwent epipolic dispersion on first entering
the liquid. It had, therefore, lost a property which it originally
possessed, and could not, therefore, be considered _qualitatively_ the
same light.--Philosophical Transactions, vol. cxxxvi.
[93] In connection with this view, the Newtonian theory should be
consulted, for which see--_A Letter of_ Mr. Isaac Newton, _Professor
of the Mathematicks in the University of Cambridge; containing his new
Theory about Light and Colors: sent by the Author to the Publisher,
from Cambridge, Feb. 6, 1671-72, in order to be communicated to the
Royal Society._
[94] In that admirable work, _The Physical Atlas_ of Dr. Berghaus, of
which a very complete edition by Alexander Keith Johnstone is published
in this country, the following order of the distribution of plants is
given:--
1. The region of palms and bananas Equatorial zone.
2. Tree ferns and figs Tropical zone.
3. Myrtles and laurels Sub-tropical zone.
4. Evergreen trees Warm temperate zone.
5. European trees Cold temperate zone.
6. Pines Sub-arctic zone.
7. Rhododendrons Arctic zone.
8. Alpine plants Polar zone.
Consult Humboldt, _Essai sur la Géographie des Plantes_, Paris,
1807; _De Distributione Geographicâ Plantarum_, Paris, 1817. Schouw,
_Grundzüge der Pflanzengeographie_. Also his _Earth, Plants, and Man_;
translated by Henfrey, in _Bohn’s Scientific Library_. Lamouroux,
_Géographie Physique_. _The Plant, a Biography_: by Schleiden;
translated by Henfrey. _Physical Geography_: by Mrs. Somerville.
[95] Fraunhofer’s measure of illuminating power is as follows:--
At the 22nd degree of the red 0·032
" 34th degree of the red 0·094
" 22nd degree of the orange 0·640
" 10th degree of the yellow 1·000
" 42nd degree of the yellow 0·480
" 2nd degree of the blue 0·170
" 16th degree of the indigo 0·031
" 43rd degree of the violet 0·0056
[96] Herschel, _On the Action of Crystallized Bodies on Homogeneous
Light, and on the causes of the deviation from Newtons scale in
the tints which many of them develope on exposure to a polarized
ray_.--Phil. Trans., vol. cx., p. 88.
[97] _On the Nature of Light and Colours_: Lecture 39, in Young’s
_Lectures on Natural Philosophy_, Kelland’s Edition, p. 373, and the
authorities there quoted.
[98] Brewster’s _Optics_: Lardner’s Cabinet Cyclopædia. Herschel, _On
Light_: Encyclopædia Metropolitana.
[99] Malus, _Sur une Propriété de la Lumière Réfléchie_: Mémoires
d’Arcueil. Numerous memoirs by Sir David Brewster, in the Philosophical
Transactions.
[100] Bartholin, _On Iceland Crystals_: Copenhagen, 1669. _An
Accompt of sundry Experiments made and communicated by that Learn’d
Mathematician_ Dr. Erasmus Bartholin, _upon a Chrystal like Body sent
to him out of Island_: in connection with which Dr. Matthias Paissenius
writes:--The observations of the excellent Bartholin upon the Island
Chrystal are, indeed, considerable, as well as painful. We have here,
also, made some tryals of it upon a piece he presented me with, which
confirm his observations. Mean time he found it somewhat scissile and
reducible by a knife into thin laminas or plates, which, when single,
shew’d the object single, but laid upon one another shew’d it double;
the two images appearing the more distant from one another, the
greater the number was of those thin plates laid on one another. With
submission to better judgements I think it to be a kind of Selenites.
Some of our curious men here were of opinion that the Rhomboid figure
proper to this stone was the cause of the appearances doubled thereby.
But having tryed whether in other transparent bodies of the like figure
the like would happen, we found no such thing in them, which made us
suspect some peculiarity in the very Body of the stone.--Phil. Trans.
for 1670, vol. v.
[101] _On the Application of the Laws of Circular Polarization to the
Researches of Chemistry_: by M. Biot.--Nouvelles Annales du Muséum
d’Histoire Naturelle, vol. iii., and Scientific Memoirs, vol. i.
p. 600. _On Circular Polarization_: by Dr. Leeson.--Memoirs of the
Chemical Society.
[102] In Sir David Brewster’s Treatise _On Optics_, chap, xviii., _On
Polarization_, the best arrangements for a polarizing apparatus will be
found described.
[103] This beautiful application was recently made by Professor
Wheatstone, the particulars of which will be found in his interesting
communication.--_On a means of determining the apparent Solar Time
by the diurnal changes of the Plane of Polarization at the Northern
Pole of the Sky_: Report of the Eighteenth Meeting of the British
Association.
[104] _On the Polarization of the Chemical Rays of Light_: by John
Sutherland, M.D., in which the author refers to the following
experiment of M. J. E. Bérard--“I received the chemical rays directed
into the plane of the meridian on an unsilvered glass, under an
incidence of 35° 61'. The rays reflected by the first glass were
received upon a second, under the same incidence. I found that when
this was turned towards the south, the muriate of silver exposed to
the invisible rays which it reflected was darkened in less than half
an hour; whereas, when it was turned towards the west, the muriate
of silver exposed in the place where the rays ought to have been
reflected, was not darkened, although it was left exposed for two
hours. It is consequently to be presumed that the chemical rays can
undergo double refraction in traversing certain diaphanous bodies; and
lastly, we may say that they enjoy the same physical properties as
light in general.”--Philosophical Magazine, vol. xx.
Dr. Leeson has stated that Daguerreotype pictures can be taken more
readily under the influence of polarized light, than by ordinary
radiation.
[105] _On the Magnetization of Light, and the Illumination of Magnetic
Lines of Force_: by Michael Faraday, D.C.L., F.R.S.--Philosophical
Transactions, vol. cxxxvii.--The following remarks are to the point
of doubt referred to in the text.--“The magnetic forces do not act
on the ray of light directly and without the intervention of matter,
but through the mediation of the substance in which they and the ray
have a simultaneous existence; the substances and the forces giving to
and receiving from each other the power of acting on the light. This
is shown by the non-action of a vacuum, of air or gases, and it is
also further shown by the special degree in which different matters
possess the property. That magnetic force acts upon the ray of light
always with the same character of manner, and in the same direction,
independent of the different varieties of substance, or their states
of solid or liquid, or their specific rotative force, shows that
the magnetic force and the light have a direct relation; but that
substances are necessary, and that these act in different degrees,
shows that the magnetism and the light act on each other through the
intervention of the matter. Recognising or perceiving _matter_ only by
its powers, and knowing nothing of any imaginary nucleus abstract from
the idea of these powers, the phenomena described must strengthen my
inclination to trust in the views I have advanced in reference to its
nature.”--Phil. Mag. vol. xxiv.
[106] The invention of the camera obscura certainly belongs to
Giambattista Porta, and is described in his _Magiæ Naturalis, sive de
Miraculis Rerum Naturalium, Libri Viginti_; Antwerp, 1561. An English
translation made in 1658 exists, but I have not seen it.
Hooke, in one of the earliest volumes of the Philosophical
Transactions, describes as new many of the phenomena mentioned by
Porta, and particularly the images of the dark chamber.
[107] Herschel, _On Light_,--Encyclopædia Metropolitana.
[108] “I would here observe that a consideration of many such
phenomena (the obliteration and revival of photographic drawings) has
led me to regard it as not impossible that the retina itself may be
_photographically_ impressed by strong light, and that some at least
of the phenomena of visual spectra and secondary colours may arise
from the sensorial perception of actual changes in progress in the
physical state of that organ itself subsequent to the cessation of the
direct stimulant.”--_On the action of the Rays of the Solar Spectrum on
Vegetable Colours, &c._: by Sir J. F. W. Herschel, Bart.
[109] Dumeril.
[110] _Theory of Colours_: by Goethe; translated by Eastlake.
[111] See Tuckey’s Narrative of the Expedition of the Zaire.
[112] The most complete examination of this subject will be found in
two Memoirs:--
1. _Experiments and observations on the light which is spontaneously
emitted with some degree, of permanency from various bodies._--Phil.
Trans., vol. xc.
2. _A continuation of the above, with some experiments and observations
on solar light, when imbibed by Canton’s phosphorus_: by Nathaniel
Hulm, M.D.--Phil. Trans., vol. xci.; and in the _Monograph of the
British Naked-eyed Medusæ_, by Professor Edward Forbes (published
for the Ray Society). See Wilson’s note to the account of _Pennalata
phosphorea_ in Johnston’s Zoophytes, 2nd edition.
[113] _A General Outline of the Animal Kingdom_: by Thomas Rymer Jones,
F.L.S.--Acalephæ, p. 64. _Lettre à M. Dumas sur la Phosphorescence des
Vers luisants_: par M. Ch. Matteucci.--Annales de Chimie, vol. ix. p.
71, 1843.
[114] _Memoirs of Benvenuto Cellini--Bohn’s Standard Library._ See also
his Treatise on his Art as a Sculptor and Engraver. Florence, 1568. 4to.
[115] _Phosphorescence of the Diamond_: by M. Reiss (Revue Scientifique
et Industrielle, vol. xxiii. p. 185).--“The diamond, phosphorescent
by insulation, lost rapidly its phosphorescence when submitted to the
action of the red rays of the solar spectrum. On the contrary, the
blue rays are those which render the diamond the most luminous in
the dark. It is probable that the phosphorescence produced by heat
is equally diminished by the action of the red rays of the solar
spectrum.” Giovanni Battista Beccaria published his experiments in
1769. See Priestley’s _History of Electricity_; and _On the Effects
of Electricity upon Minerals which are Phosphorescent by Heat_; and
_Further Experiments on the communication of Phosphorescence and Colour
to bodies of Electricity_; by Thomas J. Pearsall.--Journal of the
Royal Institution of Great Britain, Oct. 1830, Feb. 1831.--These two
memoirs contain the most complete set of experiments on this subject
which have yet been made; see _Placidus Heinrich_, _Phosphorescenz
der Körper_, vol. iv.; Gmelin’s _Handbuch der Chemie_, part 1.;--_On
the Phosphorescence of Minerals_, Brewster: Edinburgh Philosophical
Journal, vol. i. p. 137.;--_The Aërial Noctiluca, or some New
Phenomena, and a process of a factitious self shining substance_:
Boyle’s Works, vol. iv.
[116] _Des Effets produits sur les corps par les Rayons Solaires_: par
M. Edmond Becquerel.--Annales de Chimie, vol. ix. p. 257. 1843.
M. Becquerel has applied the term _phosphorogénique_ to those rays
producing phosphorescence.