CHAPTER XVII.

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.