THE WONDERS OF COLOUR AND PHOTOGRAPHY.
The young philosopher had now completed his investigations concerning the
refraction and reflexion of light. He had ascertained—
1. That all substances in nature are divisible into two classes, viz.
_luminous_ and _non-luminous_ bodies.
2. That _luminous_ bodies send off rays of light from them _in all
directions_, and that such rays proceed in _straight_ lines while
traversing the same medium.
3. That _non-luminous_ bodies are _transparent_, or _opaque_; that is
to say, they either allow the rays of light emitted by luminous bodies
to pass through them, or else they arrest their progress; sometimes, in
the latter case, driving them off from their surfaces, and sometimes
absorbing them.
4. That when a ray of light falls obliquely on a transparent body it is,
on entering it, refracted or bent, in a greater or less degree, out of
its previous straight course.
5. That when a ray of light is driven off or reflected from opaque (or
even transparent) bodies, the angle of reflexion is invariably equal to
the angle at which the ray falls upon their surfaces.
As yet, however, Humphry had dealt only with white or ordinary uncoloured
light, and he was now about to study the phenomena of colour itself—to
investigate the laws which regulate the production of the varied tints
on the earth, and to ascertain, if possible, the means by which the soil
is painted with a thousand hues, and how the colourless sunbeam becomes
broken up into countless dyes as it falls upon the flowers and the rocks,
and is driven back by them to the eye, arrayed in all the charm of
variegated lustre.
“How comes it,” said Humphry to himself, as he thought over the subject,
“that the earth by night is black and sombre, as if a pall were spread
over the dead globe—that the trees have then a dim, spectral look—that
the sky is dusky as a canopy of smoke, and that the buildings seem like
masses of dense shadow darkening the air, so that the world about us is
as colourless as a cavern, and the beauty of surrounding nature blotted
out with the universal gloom? And how is it, too,” mused the poetic
boy, “that the beams of the returning sun have power to dye the fields
and sky with the richest hues—to crimson the clouds with the glowing
tints of dawn, and to revive, as it were, in an instant, the infinite
colours of the flowers, so that the ground grows suddenly iridescent
with their various dyes? How comes it, again, that at the tropics, where
the sun steeps the earth in a flood of light, the plumage of the birds
and the blossoms of the plants are unrivalled for the gorgeousness of
their colours; and that as we proceed thence to colder climates we find
a regularly declining chromatic scale, the tints becoming less and less
vivid till we reach the Poles, where Nature is arrayed in one unvarying
robe of white?”
But Humphry was too anxious to experiment to continue dreaming over the
matter, and, accordingly, he got his sister to darken the room once
more, and then attaching a prism in front of the hole in the shutter,
he proceeded to throw the spectrum on the wall in the manner before
described.
Kitty was as delighted as the boy with the beauty of the image, nor could
she help wondering how it happened that a simple stick of white glass
could resolve the sunbeam into such exquisite tints.
Humphry told her that the image she saw on the wall was merely an oblong
picture of the sun itself, the orb being drawn out to that figure by the
refraction of the glass. The vividness of the colours depends upon the
smallness of the aperture through which the light is admitted, and the
distance of the screen upon which the spectrum is made to fall; so that
if the hole in the shutter were smaller, and the wall farther off, the
spectrum would be much brighter. The colours, he added, came from the
decomposition of the sunbeam into its elementary tints.
“You, doubtless, Kitty, think the light of the sun to be simple and
uncompounded, and little dream that every ray of white light which meets
your eye is made up of seven other beams, and each coloured with some
one of the tints you see in the spectrum here; and it is solely because
the sunbeam is a _compound_ rather than a simple thing, and that each
of the seven rays of which it is composed have different properties and
refrangibilities, that this glass prism has the power of separating
them one from the other, and so of resolving the compound beam into its
seven elementary rays. Look here,” he continued; “were it not for this
prism the beam which comes through the hole in the shutter would proceed
in its previous course, and strike upon the floor; but by means of
this instrument it is _refracted_, or bent out of its path, and as the
coloured beams of which it is composed are, as I said, all differently
refrangible, the red ray here, being the least refrangible of all, is
the least bent out of its course, and so made to appear at the bottom
of the spectrum; whereas the violet ray, which is the most refrangible,
undergoes the most deviation, and thus is found at the upper part of the
coloured image.”
Kitty acknowledged that it was beyond her power to comprehend how a
beam of white light, which appeared to her to be devoid of all colour
whatever, could be _really_ composed of every kind of colour. “How was it
possible,” she said, “for violet, and blue, and green, and yellow, and
red, when mixed together, to form white?”
Humphry smiled at his sister’s incredulity, and said he would show her
that it was quite possible to put the parts of the sunbeam together
again—for by the same means as he had decomposed the white beam into its
seven coloured rays, so would he compound those seven coloured rays again
into one colourless beam.
The girl was all eagerness to see the composite nature of light thus
practically demonstrated, and in obedience to her brother’s instructions
she proceeded to place a sheet of white pasteboard against the wall, so
that the spectrum might fall upon it, and then to bring it gradually
nearer the prism.
As Kitty did this, she noticed that the spectrum grew smaller and dimmer;
but though the colours began to mix and encroach upon one another as she
advanced towards the prism, she found that, even when the pasteboard
screen was brought close to the face of the glass she could still
recognise the separation of the light into its elementary coloured beams.
This done, Humphry proceeded to annex to the prism already employed
another, which was exactly similar in all respects—being made of the like
kind of glass, and having a like refracting angle—to the previous one.
The second prism, however, was placed in the opposite direction to the
first, so that while the base of the one was uppermost, that of the other
was underneath—as here shown:
[Illustration]
The reason of this arrangement was, as Humphry explained, that the second
prism might exactly undo what the first had previously done, so that
the rays being now refracted by the one in an opposite direction to the
other, they would be all brought together again, and made to strike upon
the same spot as they would have fallen upon had no such instruments been
interposed.
The apparatus being fixed, the ray from the hole in the window-shutter
no sooner passed through the two prisms than the coloured spectrum which
the beam had been previously resolved into vanished from the wall, and a
round white spot of light appeared upon the floor.
Kitty was so wonder-stricken at what she saw, that she looked at Humphry
with the same fixed stare as a child gazes at some parlour magician.
“You see, then, sister,” said the lad, “that seven coloured rays may
be compounded again into one white one, even as one white beam can be
decompounded into seven coloured ones. So that, incredible as it may seem
to you, it is impossible to avoid the conclusion that white light is a
_composite_ thing, and made up of a number of other kinds of light that
are widely different from it. But,” continued Humphry, “there is another
and simpler means of proving this point to you.”
For this purpose the girl had to procure from the colour-shop seven
different colours in powder, each of the same tint as one of the rays in
the spectrum. These were afterwards mixed in the same proportion as the
rays themselves bore to one another, and, to Kitty’s astonishment, the
result was a kind of _greyish white_, produced by the mingling of the
whole; and Humphry told her that, were it possible to obtain colours of
precisely the same tint as those in the spectrum, a perfect white would
be the consequence.
“Again,” the youth continued, “if we take a circle, and paint round it
the several prismatic tints in the same proportion as they exist in the
spectrum itself, and cause these to revolve so rapidly that the eye is
unable to see any _one of them_, but rather perceives the whole at once,
the paper will no longer appear coloured like the rainbow to us, but seem
really white as it flashes past the eye.”
It was necessary, however, before doing this, to measure the several
lengths of the coloured spaces in the spectrum itself, when it was found
that the various prismatic tints were in the proportions hereunder given:
[Illustration]
The next step was to colour a circular piece of paper, as nearly as
possible in the same manner as the spectrum, and this was done after the
following fashion—where it will be seen that the outer coloured circle
is nothing more than the prismatic spectrum bent round till its two ends
meet at the point between the violet and red—the entire circle itself
being supposed to be divided into 360 parts.
[Illustration]
The circular spectrum, when finished, was placed upon a humming-top, and
the top being made to spin as rapidly as possible, the prismatic disc,
as it whirled past the eye, appeared to be absolutely colourless; for
each tint as it revolved left its impression upon the retina but for an
instant, and this being immediately afterwards covered by the tint which
was next to it, the result naturally was, that the whole of the seven
colours fell upon precisely the same part of the retina itself, and so
produced a composite impression—the seven coloured rays being perceived
all at once, rather than one after another. Hence the circle seemed to
be devoid of any _one_ of the colours painted upon it, and to partake of
that white tint which naturally results from the blending of the whole.
Humphry himself was almost as delighted as his sister with the result
of his experiments. It was demonstrable that the light of the sun which
fills the air by day, and seems absolutely colourless to us, is not of
that simple homogeneous nature which we are naturally led to believe, but
really made up of _seven_ coloured rays, which the eye itself is unable
to separate, and from which proceed all the several hues with which the
earth is painted, for the composite white beam falling upon the different
objects around is broken up by them into its elementary tints, and some
one of these reflected by them to us, so that the object itself naturally
appears of the same colour as the beam it sends to the eye.
* * * * *
Humphry had now to investigate the several properties of the spectrum
itself.
It will be remembered that he before found the point of greatest _heat_
to exist at the very extremity of the red ray, and he now ascertained
by means of a _photometer_ (or an instrument for measuring the relative
intensity of different lights) that the point of _greatest light_
existed at the boundary of the orange and the yellow rays. Consequently,
as the red (or calorific) rays were less refrangible than the yellow
(or luminous) rays, there was but one conclusion to come to—_light was
itself more refrangible than heat_; that is to say, the light in passing
through the prism, and being there separated from the heat with which it
was previously associated, was bent farther out of its course than the
heat was, so that the two principles were differently acted upon by the
glass, and consequently possess different powers and susceptibilities.
“But if,” said Humphry, “the red rays are the calorific ones, and the
yellow rays the luminous ones, what peculiar properties belong to the
rays at the upper end of the spectrum, where the sunbeam is bent the
farthest of all out of its previous course? What special power appertains
to the violet and blue portion of the coloured image?”
The youth knew, from the books he had studied upon the subject, that
these constituted the chemical beams—that is to say, the violet extremity
of the spectrum had been found to possess the power of separating silver
from some of its compounds, and Humphry was now anxious to observe the
effect for himself.
Having brushed a paper over with a solution of _nitrate of silver_
(lunar caustic), he placed a strip of it a little way beyond the violet
extremity of the spectrum; another strip he deposited in the violet ray
itself; a third was left in the blue ray; while in each of the other
coloured portions a piece of the same paper was exposed, and the light
admitted to them all at the same time.
It was then found that the nitrate of silver darkened the _most_ rapidly
at that part of the spectrum a little _beyond the violet extremity_—that
the chemical effect was the greatest after this in the violet ray itself.
Next, the blue ray possessed a greater decomposing power than the green;
whilst in the yellow and red rays no such power was perceptible, for the
solution of silver remained undarkened there.
“So, then,” cried Humphry, “the wonderful sunbeams that stream every
day upon the earth contain not only all the colours of the rainbow, but
three distinct, subtle principles, locked up in them—heat, light, and
chemical influence; each of these being differently refrangible and
existing in a ray of a different colour; the heat inhering in the red
or lower portion of the spectrum, and the chemical power in the violet
or opposite extremity, whilst the light occupies, as it were, a middle
place, residing principally in the yellow portion.”
Humphry then delighted his sister by preparing different chemical
solutions, to be acted upon by the violet rays of the sun.
First, he made some _chloride of silver_ by steeping a paper in salt and
water and then brushing it over with a solution of _lunar caustic_. This
he found to darken even more rapidly than the nitrate of silver itself,
and he then set to work to ascertain the cause.
Now _chloride of silver_ he knew to consist of chlorine (a green coloured
gas) and silver, and he was anxious to see whether light would act upon
chlorine more powerfully than it did upon nitric acid, as Mr. Wedgwood
had told him.
Accordingly he filled a jar with equal portions of hydrogen and chlorine
gases, and submitted this to the action of the sun’s rays, when, to the
astonishment of himself and terror of Kitty, the jar was no sooner placed
in the sunshine than the two gases detonated with the noise of the report
of a pistol, and the jar itself was almost shivered to pieces in the
explosion.
Delighted with the result, and anxious to repeat the experiment in a less
dangerous form, he filled a tube, about half an inch in diameter and
twelve inches long, with the same gases, and while the end of the tube
was inserted in a vessel of water, the upper part of it was shaded with
an opaque cover, so that by removing this for an instant he could allow
the gases within the tube to be acted upon by the light for as short a
time as he pleased.
In this manner the ingenious youth found, that the moment the opaque
cover was removed and the tube exposed, even to the diffused light of
day, a cloudiness appeared within it, owing to the instantaneous, though
silent, combination of the two gases, while the water rose more or less
rapidly within it according to the intensity of the light. The effect
even of a passing cloud was thus distinctly seen to retard the rapidity
of the combination, while, when exposed to the full solar light, the
union of the two was so instantaneous that the gases suddenly disappeared
from the tube, and the water rushed violently up into it to fill the
vacuum.
Next Humphry found that the two gases, when exposed to the sun’s rays
in a tube of violet-coloured glass, combine rapidly, but, strange to
say, without explosion; whereas when they are submitted to the action of
sunlight in a tube of red glass, the gases scarcely act upon one another.
It was, moreover, ascertained, that when standing in a perfectly dark
place, the two gases do not enter into combination in any length of time.
The lad could now understand why the chloride of silver darkened so
rapidly in the sun’s rays. The chlorine with which the metal was combined
was attacked by the moisture in the atmosphere, and as this moisture
consisted of oxygen and hydrogen in the form of water, the hydrogen of
it was made by the chemical influence of the sunbeam to enter into rapid
combination with the chlorine, and thus the silver was left behind, but
in such minute particles that the metal, instead of appearing white as it
usually does, assumed the form of a black powder, which, being fixed in
the paper, naturally caused it to darken in those parts where the light
had fallen upon it.
Filled with the knowledge he had thus obtained, Humphry set to work to
produce some sun-pictures for his sister. Patterns of pieces of lace were
thus made to impress their forms in a few seconds upon paper that had
been prepared over-night with a coating of chloride of silver. Where the
light fell, the silver was separated from the chlorine, and precipitated
in minute black particles, so that the paper was darkened in those parts;
while in the places where the threads of the lace prevented the rays from
reaching the paper, the solution was undecomposed, so that a white line
exactly corresponding to the pattern of the lace itself became impressed
upon the black ground.
Kitty was overjoyed at the first picture she beheld her brother produce
by the light, and Humphry smiled as he saw her take it to the window to
examine it more minutely, for he knew that as she looked at it the light
would begin to act upon the parts that had been previously screened by
the lace itself, and where the solution still remained undecomposed in
the paper; and sure enough, in a few minutes, she gradually saw the
pattern vanish, and the whole ultimately become of one uniform dark-brown
tint.
“What a pity,” cried the girl, “that so beautiful a thing should be
so perishable! If you could only find out, Humphry, how to fix the
pictures, what a great thing it would be for you to do!”
The brother told her, that in order to accomplish this it was necessary
to discover some substance that would remove the undecomposed chloride
of silver, forming the white parts of the picture, and which would not
attack the decomposed silver itself, forming the dark parts of it.
To attain this end, the young chemist made an infinity of experiments,
but without avail; for though he tried a number of acids and alkaline
solutions, he could find no liquid that would remove the undecomposed
chloride from the paper, and after weeks of toil and disappointment he
was obliged to confess, unwillingly, that the difficulty was one he
lacked the power to master.
Many years after Davy’s time, however, it was discovered that the
chemical substance termed _hyposulphite of soda_ readily dissolves
chloride of silver, and has little or no action upon the precipitated
silver itself; and from this period may be dated the perfection of the
wonderful art of photography (or sun-painting) that Thomas Wedgwood was
the first to attempt, and at which Davy himself was one of the early but
unsuccessful experimenters.
Of this art there are now two distinct branches, viz. one in which the
pictures are produced upon metal, the other upon paper or glass. In the
metallic process, _iodide of silver_ is the chemical agent rather than
the _chloride_; this is formed by submitting a perfectly clean plate of
polished silver to the action of the vapour of iodine, and sometimes to
_bromine_ afterwards, in order to quicken the action. The plate thus
prepared is placed in the camera, so that an image of the object to be
copied may fall upon it; the consequence is, that in the “_lights_” of
the picture the _iodide of silver_ becomes decomposed, the iodine itself
going off in the form of _hydriodic acid_ gas, by combining with hydrogen
in the moisture of the air, and the pure silver being left behind;
whereas in the shades of the picture where no light reaches, the iodide
of silver remains undecomposed. The action usually takes place in some
few seconds, according to the intensity of the light and the nature of
the “quick” used. When the plate is removed from the camera no picture is
visible upon its surface, so that the _developing_ part of the process
has then to be performed. This consists in submitting the plate to the
fumes of mercury, which attach themselves to the parts where the pure
silver has been separated from the iodine with which it was combined.
These parts constitute, as we said before, the “lights” of the picture,
and there the mercurial vapour is condensed, and clings in the form of
minute globules; whilst to the parts which have been undecomposed the
mercury does not attach itself, having no affinity whatever with the
_iodide of silver_ that remains there. The consequence is, the globules
of mercury which cling to the portions where the rays have fallen,
reflect so much light to the eye that they form the “whites” of the
picture; whereas the undecomposed iodide of silver, sending no light
to the retina, constitutes the “blacks:” and thus the image, which was
latent on the plate, is developed, or brought out, with such marvellous
fidelity, that when examined with a microscope, characters that were
several miles distant in the original may be clearly read in the minute
sun-copy.
After this comes the fixing process, and that consists merely in
submitting the plate to the action of _hyposulphite of soda_, which
dissolves, and so removes all the undecomposed _iodide_ of silver from
it, and thus renders it incapable of being further acted upon by the
light.
The above constitutes what is now usually known as the “_Daguerreotype_
process.”
The production of photographic pictures upon paper, on the other hand,
forms what is termed the “_Talbotype_ process”—the names of the two types
being derived from, those of their inventors. In the latter method of
producing sun-pictures there are almost the same different stages to
be gone through. The paper itself has first to be iodised, or rendered
_sensitive_ to the action of light, by means of coating it with a surface
of iodide of silver. This is done by washing it over first with a
solution of _nitrate_ of silver, and when this is dry, with a solution of
_iodide of potassium_; the consequence is, the one solution decomposes
the other, so that nitrate of potash and iodide of silver are formed. The
nitrate of potash, being soluble, is then washed out of the paper, while
the insoluble iodide of silver remains fixed in it. Then follows the
“_quickening_” part of the process. This consists in washing the sheet
of iodised paper over with a solution of what is termed _gallo-nitrate_
of silver, which consists of a small proportion of gallic acid (the acid
from gall-nuts) dissolved in water, and added to a solution of lunar
caustic, having a little acetic acid, or pure vinegar, in it. The gallic
and acetic acids are used because it is found that the presence of any
vegetable or organic matter hastens the decomposition of nitrate of
silver when exposed to light. The paper is now ready for the camera, and
is so sensitive to the action of light that it is said to transcend the
ordinary iodised paper in this respect more than a hundred-fold, so that
even a second or two of time is sufficient to impress a latent image upon
it.
Then, as in the daguerreotype method, the _developing_ process has to be
resorted to in order to bring out the picture, which is imperceptible on
removing the paper from the camera, and the existence of which would not
be suspected by any one who had not been forewarned of it by previous
experiments. To render the picture visible, the paper is washed over
once more with the gallo-nitrate of silver before described, and then
warmed gently before the fire; whereupon that part of the paper upon
which the light has acted begins to darken, while the other part of
the paper retains its whiteness. After this, as in the “Daguerreotype”
method, the _fixing_ process has to be resorted to. This, for
“Talbotypes,” consists in washing the paper in _bromide of potassium_,
which dissolves out all the undecomposed chemicals, and so leaves an
indelible impression behind.
The picture thus produced, however, is what is termed a “negative”
one—that is to say, the lights in the original are represented by shades
in the photographic copy, and _vice versâ_, the shades in nature are
rendered as lights in the picture. The Talbotype, therefore, has to be
again copied, in order that the lights and shades may be accurately
represented. For this purpose, however, the paper need not be so highly
sensitive; so that the ordinary _quickening_ part of the process by
means of the gallo-nitrate may be dispensed with, or the paper may be
coated with chloride of silver instead of the iodide before described.
Again, the _developing_ process is no longer necessary, the picture being
produced _directly_ by the action of light, rather than indirectly by
means of some developing agent. The _fixing_ process in this stage is
usually performed by means of hyposulphite of soda, and by these means
the negative picture before produced is rendered positive, and the lights
and shades thus made an accurate representation of those in nature.
It will now be seen that the art of producing sun-pictures, whether by
the Daguerreotype or the Talbotype, comprises usually four distinct
processes, viz.:
1. The _preparatory_ process, which consists in preparing the plate or
paper—that is to say, in coating it with some solution of silver that is
capable of being decomposed by the action of light.
2. The _quickening_ process, which consists, again, in rendering the
plate or paper more highly sensitive to light by the addition of some
other chemical, which facilitates the decomposition of the compound of
silver, with which the surface has been previously coated.
3. The _developing_ process, which consists in rendering visible the
latent picture which has been impressed upon the plate or paper while
exposed to the action of the light in the camera.
4. The _fixing_ process, or the dissolving out of all the undecomposed
silver compound, and so preventing the light from having any further
action upon it.
Now it must not be supposed that the compounds of silver are those only
which are capable of being decomposed by the sun’s rays, for photographic
pictures have been produced by compounds of all the precious metals—such
as gold, platinum, mercury, &c., these substances having but slight
affinities, and so being easily separable from the elements with which
they are united. Again, iron has been used successfully for the same
purpose—for this body, also, is readily decomposed when combined with
certain substances. Further, the gum-resins and bitumens admit of being
employed in the same manner, and many vegetable juices have been used by
Sir John Herschel for a like purpose. Indeed it has been truly said, that
almost every substance in nature is affected, in some way or other, by
the solar rays, for we now know that no substance can be exposed to the
sun’s rays without undergoing chemical action.
The changes, therefore, that are continually occurring in the external
world are quickened by the rays, which at one time it was believed gave
only light and heat to the globe that we inhabit; and even the very
changes of the seasons, the growth of vegetation, the blossoming of the
flowers, and the ripening of the fruits, are all due, in a measure, to
the chemical influence of those elementary rays which lie concealed in
the compound sunbeam: for it has been proved that the sunshine itself is
necessary, even to the breathing of plants through their leaves, and
that in the shade they cease absorbing the carbon from the atmosphere
which is ultimately destined to form part of their woody structure. Thus
light becomes not only the source of beauty to the world, and the agent
upon which one of our most wondrous senses depends, namely, that by which
we are enabled to recognise the form and nature of objects at a distance
from us, but it is also the source of health and vigour to our frames,
by maturing the products of the earth upon which we live, as well as by
promoting in our own frames those subtle chemical changes, by which our
bodies are nourished and our faculties developed, since in darkness men
can no more thrive than plants.
Had Davy lived to see the development of the chemical influence of light
that has been opened up to us since his time, he would have been the
loudest in his praises of the marvels wrought by it, and, doubtless,
among the foremost to have extended our knowledge of its action. But
these are discoveries made since his time, and discoveries which he, with
his deep insight into Nature, was unable to foresee, or even to assist.
To such perfection, however, has the photographic art been carried since
the days when Davy vainly essayed to fix the images which took him some
quarter of an hour to produce, that not only can stationary objects have
their forms indelibly impressed upon paper by the very light itself
which renders those forms visible to us, but the passing shadows that
give beauty to the landscape can be made permanent, the very undulation
of the corn can be seized, the rustling of the leaves detained, and even
the rippling of the waters, the playing of the fountains, and the curling
of the smoke, whose particles never for two moments together remain in
the same place, can be arrested, and their evanescent forms painted by
themselves, as it were, upon the tablets; so that the effect of a mere
instant can, by its marvellous agency, be prolonged for years. Thus time,
which is known to us only by the changes which are continually occurring
without and within us, has all the fixity of space; and those historical
events which our forefathers were unable to convey to us, from the want
of some such art, can now be handed down, rendered with all the truth of
light itself, so that future generations gazing at them may behold the
same scenes that were impressed at the back of our eyes years before.
Indeed, the photographic art itself is but the process of individual and
transient vision made universal and permanent; for the eye itself is but
the camera through which we gaze at the world without, and the retina
at the back of the organ of sight no more than a photographic plate, as
it were, impressing the images that flit before our vision more or less
permanently upon our memories.
As an instance, however, of the perfection, we repeat, to which this
process of fixing the most transient images has been carried, we need
only mention the experiment performed by Mr. Talbot, in which a moving
body, that was made to revolve at an enormous speed, and that was
illuminated but for an _instant_ by the electric spark, was photographed
as a stationary object.
It is well known that a wheel revolving at a rapid rate is barely visible
to us, the spokes passing with such velocity before the eye that we are
unable to distinguish one from the other, so that the whole appears to
us almost as one entire disc; such a wheel, however, if made to rotate
in the dark, and then suddenly illuminated for that inappreciable
portion of time which the electric spark—the miniature lightning of the
laboratory—endures, then appears to us as if absolutely standing still;
for as we see it under such circumstances only in that place which it
occupies so long as the light lasts, and this being but for the least
conceivable term of duration, it has no time—however rapidly it may be
turning on its axis—to pass from one point of space to another, so that
it can but appear to us as if utterly stationary.
In the experiment we allude to, a wheel was thus made to revolve so
rapidly, that its revolutions were counted by the musical note produced
by the vibrations of a spring, that moved backwards and forwards once at
each turn. The revolutions were performed in the dark; and during this
the chamber and wheel were suddenly lighted by one spark drawn from a
powerful electric machine. At this moment a photographic apparatus was
presented to the wheel itself, and on developing the image thus produced
upon the paper which had been previously inserted in the camera, it was
found to be impressed with a perfect copy of the wheel itself, with all
its spokes distinctly visible, and precisely the same as if the image
had been taken from the wheel while in a state of rest. It was the same
stationary image, too, as the spectators themselves had beheld during
the instantaneous illumination of the object; and thus, by the aid of
the same fluid as the lightning itself, and with the assistance of music
to register the rate of revolution, that mysterious principle of motion
which has puzzled philosophers since philosophy began, was made to
appear like rest, and even the sensibility of the eye itself rivalled by
photographic agency, so that the dead paper was made to be impressed with
the very same figure as the living retina itself perceived.