Sunday, October 29, 2017

Synthetic Reconstruction of the Elements of an Analyzed Movement -- October 29, 2017

Étienne-Jules Marey was a French scientist who pioneered chronophotography, the capture of phases of movement in photographs.  His 1895 book Motion describes his years of work.  This chapter talks about reconstructing motion from the photographs using a phenakistoscope and a zoetrope.  

Joseph Plateau was a Beligian mathematician.  Ottomar Anschutz created the electrical tachyscope, a form of zoetrope.  Charles-Émile Reynaud created the Praxinoscope and Théâtre Optique. Georges Demenÿ used his Photophone to analyze mouth movements in speech.

I like the 3-D zoetrope. 



Summary.—Plateau's method; his phenakistoscope The zootrope; its applications to the study of horses' paces and their relations to one another—The use of instantaneous photography in connection with the zootrope — Muybridge, Anschütz— Scientific applications of Plateau's method—Points of a good apparatus— Improvements made by different authors—Attempts at constructing a chronophotographic projector.

Although chronophotography represents the successive attitudes of a moving object, it affords a very different picture from that which is actually seen by the eye when looking at the object itself.

In each attitude the object appears to be motionless, and movements, which are successively executed, are associated in a series of images, as if they were all being executed at the same moment.

The images, therefore, appeal rather to the imagination than to the senses. They teach us, it is true, to observe Nature more carefully, and, perhaps, to seek in a moving animal for positions hitherto unnoticed.

This education of the eye may, however, be rendered still more complete if the impression of the movement be conveyed to the eye under conditions to which it is accustomed. Such is the object of Stroboscopy, a method of immense scientific importance. The principles of this method were discovered by Plateau, and they depend on the physiological property of the retina of retaining for a brief moment the impression of an image after the object which has produced it has disappeared. The duration of this retinal picture is estimated at part of a second. So that if an image is placed before our eyes ten times in a second the idea of discontinuity is lost, and the images appear to be in continual evidence.

If the images shown to us are represented in the successive positions assumed by the object in motion, the impression conveyed to the eye is that of a continuous movement with no intermission. Now, we have seen that not only 10, but even 20, 40, or 60 images can be produced by chronophotography per second. If the 60 photographs taken during one step of a galloping horse could be passed before the eyes at the rate of 10 per second, the duration of the whole step would be spread over a period of 6 seconds, and hence we should have a considerable time in which to observe the motion of the limbs, so hard to follow under normal conditions.

In the same way a flying bird could be represented with slower wing movement, and so too with other phenomena which escape notice on account of their extreme rapidity.

Inversely, when a movement is so slow as to escape observation, photographs could be taken of it at long intervals and presented to the eyes in sufficiently rapid succession to allow of the changes being clearly perceptible. In other cases, if the photographs are presented to the eye at the same intervals as separate the successive exposures, the movement will appear as it actually took place. Such is the use of the stroboscope. We will now show the successive developments of this method.

Plateau's Phenakistoscope. — Everybody knows the ingenious toy invented by Plateau at the beginning of the present century, and to which he gave the name of "Phenakistoscope."

The original form of this instrument was a plaything which delighted us as children; it was destined, however, one day, to be used for more interesting purposes. The phenakistoscope consists of the following parts, a cardboard disc perforated at equal distances round the periphery by small slits. One side of the disc is blackened, and on the other a series of images are arranged representing men or animals in the various attitudes which correspond to the successive phases of a movement.

When the disc is spun round on its axis opposite to a mirror, and the eye applied to the blackened side on a level with the revolving slits, the reflections of the various images are seen one after another corresponding to the different attitudes assumed by the original object; this conveys an impression of actual movement. Fig. 202 represents the disc of a phenakistoscope; on it are arranged the successive photographs of a flying gull.

If this side is made to revolve in front of a mirror and the eye be applied on a level with the slits, the gull can be seen flapping its wings. The rapidity of the movement depends on the velocity of rotation.

The disc must be turned in the right direction, otherwise the images will succeed one another in the inverse order to that in which they actually occur, and the direction of movement will appear reversed.

Zootropes.—The manufacture of these articles became a commercial industry, and some of them were turned out in more convenient forms; one of them was called the "zootrope," and consisted of a cylindrical chamber revolving on a vertical axis. Narrow upright slits were made round the brim, and inside the cylindrical wall a strip of paper was pasted on which a series of images was arranged so as to represent the successive attitudes of a man or animal in motion. If these figures were observed through the slits while the zootrope was revolving, the same impression as that caused by the phenakistoscope was produced.

This contrivance, which has been adopted by many manufacturers, possesses one obvious advantage, namely, that several people arranged round the apparatus can watch the phenomenon at the same time.

Application of the Zootrope to the Study of Horses' Paces.—In the year 1867 we made use of the zootrope for the purpose of representing the various paces of a horse in motion, and also for showing how the various paces differed from one another. The latter could be shown by merely altering the sequence of the movements of the fore and hind limbs. This was the concrete demonstration of the sequence expressed by the chronographic charts.

At this time instantaneous photography had not been thought of, and so we used simple drawings to show the successive positions, our data were derived from the registered charts and from the actual foottracks.

We chose first the simplest case, namely, the paces of an ambling horse, in which the two limbs on the same side acted simultaneously. Twelve positions were drawn on a long strip of paper, six to represent the rise of the two feet on the right-hand side, the other six to represent their period of contact on the ground, the two feet on the left-hand side were of course in the opposite phases.

By arranging this strip of paper in the zootrope, the paces of an ambling horse could be easily recognized through the slits.

Now, for the purpose of showing how the other paces could be derived from those of ambling, we had recourse to the following device. Vertical lines were drawn through the middle of the horses' bodies, and square frames were constructed round the posterior halves of the figures containing the hind limbs of the animals. The squares of paper were then cut out, and the original strip of paper then remained, representing a series of positions of the fore quarters, and behind each of these mutilated images there appeared a square hole in the paper. The strip of paper was then placed on another of the same size, and the hind portions of the images which had been cut away were gummed each in its proper position upon the lower strip. When this was done, the two strips taken together presented the appearance of the original slip, namely, the successive attitudes of an ambling horse during the performance of one stride. If the lower strip is moved on one place, so that the fore feet of one image are united to the hind feet of the image immediately behind it, the fore feet will be ^ of a step in advance of the hind feet, and the whole series thus broken up will give the appearance in the zootrope of a racking pace, in which the hind limbs slightly anticipate the movements of the fore limbs.

By sliding the lower strip of paper a little further forward the appearance of a walking pace is produced. Still another move in the same direction and we have a broken trot, and then again a walk.

This is the concrete expression of the relations given by the chronographic chart Fig. 123.  (I omitted the chart from this post -- JT)

With this method persons familiar with horses' paces can recognize each example, and realize its derivation from the others. We have been most ably seconded in these researches by M. Mathias Duval, now professor at the Faculty of Medicine and at the School of Fine Arts. This savant recognized the importance of this method for teaching complicated and rapid movements such as could otherwise only be learned at the cost of much labour by specialists. M. Zecky, professor at the School of Fine Arts at Vienna, has adopted the same method for representing horses' paces. We still possess some very carefully drawn series which he sent us.

Use of Instantaneous Photography in connection with the Zootrope.—In this method the accuracy with which the paces were represented was entirely dependent on the skill of the artist, and hence it was left for photography to perfect the zootropic representation of motion.

From the time when Mr. Muybridge succeeded in taking a photographic series of the positions assumed by men and animals in motion, he invariably resorted to Plateau's method for synthesizing the movements he had analyzed.

The apparatus used by Mr. Muybridge was a sort of projection phenakistoscope, in which pictures of horses painted on glass discs, and copied from the author's photographs, were placed in the focus of the projecting lantern and made to rotate. Slits made in the discs admitted light at the required moments. A considerable audience could thus see upon the screen silhouettes of horses moving in various directions and at various paces.

Zootropes of Muybridge and Anschütz.—We have already remarked that the figures were painted. Now, one great disadvantage of Muybridge's apparatus, and, indeed, of the zootrope itself, is that the figures are out of proportion, owing to their reduction in the transverse direction, so that the painted horses on the revolving discs have to be made longer than they really are, so as to appear in their true proportions when thrown upon the screen.

M. Anschütz prepared for the ordinary zootrope strips of paper covered with photographic prints of men and animals in motion. In this case the figures were distorted; horses especially showed an appreciable diminution in length.

Solid Figures in the Zootrope.—We also made use of the zootrope in studying the movements of birds' wings, and for this purpose we resorted to a particular contrivance. Instead of a strip of paper covered with figures, we introduced into the zootrope a series of wax models painted in oils, and representing the bird in all the successive phases of its wing movement. The illusion was complete, and a flying bird could be seen flying round and round the apparatus; sometimes flying away from the observer, sometimes across, and sometimes towards him. (This zootrope with solid figures is still preserved at the Physiological Station.)

Scientific Applications of Plateau's Method.—All these applications would be simply childish if they were limited to the reproduction of phenomena which could be observed by the eye in the case of living creatures. They would be attended, in fact, by all the uncertainties and difficulties which embarrass the observation of the actual movement. In a bird, for instance, the wings could only be distinguished as an indistinct mass, just as they appear in nature. But a combination of the zootrope and chronophotography has further possibilities, for it enables the observer to follow movements, which would otherwise be impossible to examine, by slowing down the motion to any desired rate. We have already pointed out during a single stroke of the wing, which lasts 1/5 of a second, a series of twelve photographs can be taken at intervals of 1/60 of a second. Now, these twelve photographs, which correspond to a single stroke of the wing, can be made to pass before the eye in one second. This succession is sufficiently rapid to produce an impression of continuous motion. Under these conditions, the rate of movement is reduced to one-fifth of its actual velocity, and the eye can follow it in all its phases, whereas, in a living bird, only a confused flutter of the wings can be distinguished.

In the same way, by slowing down the phases of a horse's paces by means of the zootrope, they can be more easily analyzed than by observations made directly on the animal.

It is not, however, only by reason of their rapidity that some movements elude observation, sometimes their very slowness renders them inaccessible to our senses, take, for instance, the growth of animals and plants. These movements may, however, become quite visible if they are photographed at considerable intervals of time, and the corresponding series of images passed rapidly before the eyes by means of the zootrope.

Professor Mach, of Vienna, suggests a curious line of research by means of this method. His idea is to take a number of photographs of an individual at equal intervals of time, from earliest infancy until extreme old age, and then to arrange the series of images thus obtained in Plateau's phenakistoscope. If this were done, a series of changes, which had been brought about during a period of many years, would pass before the eyes of the beholder in the course of a few seconds, and thus the stages of a man's existence would pass in review before the gaze of the onlookers in the form of a strange and marvelous metamorphosis.

This method invented by Plateau seems likely to extend our knowledge as regards all kinds of phenomena. But the future of the method is dependent on the possible correction which can be effected in the distortion of the images, and on the discovery of a satisfactory means of projecting a number of moving figures on a screen, so as to be visible to a large audience. And, further, it will be necessary to augment the number of successive photographs, so as to represent a performance of considerable duration.

Improvements suggested by Different Makers.—So that the images might be projected without distortion, several object-glasses were arranged in a circle, and at the focus of each positive images were arranged representing the different phases of a movement. All the object-glasses were directed towards the same spot on the screen in such a way that by successively illuminating each of the positive images placed behind them, the corresponding attitudes were successively projected on the screen. To effect the successive illuminations of the slides, it has been suggested that a Drumont lamp should be made to revolve as a source of light.

Images projected in this way ought to be perfect; but the focusing of each, and the determination of the direction of the object-glasses, would be a most laborious operation. Moreover, the number of object-glasses is necessarily limited to five or six, and thus the extent of the movement periodically repeated, as the lamp completes its revolution, is necessarily very short.

M. Raynaud's Praxinoscope.—Under this title, M. Raynaud (Reynaud - JT) has given to the world an extremely ingenious instrument. As in the ordinary zootrope, the figures are arranged within a cylinder, and are reflected by a prismatic mirror situated at the centre of the apparatus, and thence reach the eye of the observer. The contrivance is peculiar in that the substitution of one position for another is effected without any intermediate eclipse, so that the images, owing to the constant illumination, appear exceedingly bright. By interposing a photographic object-glass in the path of the reflected images, M. Raynaud has thrown them upon a screen, and magnified them to the required dimensions. Finally, by substituting for the flat circular strip of figures a long strip which winds off one roller on to another, the writer has been able to display a performance of considerable duration. As yet, M. Raynaud has only employed figures' drawn or painted by hand; doubtless he could obtain remarkable results by substituting a series of chronophotographs. A slight defect in the apparatus is that the plane of the projected images is slightly oblique as regards the principal axis of the object-glass, this is due to the construction of the apparatus. It is thus impossible that all the parts of the images can be in focus, and hence the projection on the screen is somewhat indistinct.

M. Demeny's Photophone.—M. Demeny has employed another method for reproducing the movements of the face, the tongue, and the lips, executed during speech. My assistant at the Physiological Station has prepared on a length of film a chronophotographic series consisting of twenty-four portraits of a man articulating certain words. When this series of portraits was transferred to the circumference of a glass disc and placed in the focus of a photographic object-glass, they were brightly illuminated from behind, and rendered visible for brief intervals by an arrangement of fenestrated diaphragms, as used in chronophotography. The shortness of the exposures, and the perfect working of this apparatus, represented the images as immovable, and they appeared exactly at the same spot, in spite of the rotation of the disc upon which they were placed. (This instrument was designed by M. Demeny, and called the "Photophone" (C. R. de l'Academie des Sciences, July 27,1891).

These photographs give such a perfect representation of speech that deaf mutes accustomed to read the movements of the lips have been able to recognize the words spoken by the person photographed.

I doubt whether it is possible to make a more perfect zootrope, and yet there are a few defects that one can mention. Firstly, the number of images that can be transferred to the disc is necessarily limited, unless the apparatus is of enormous size; and, secondly, since a good definition of the movements can only be obtained by very brief exposure, it follows that the amount of light given off must be too small to produce with distinctness an enlarged projection, and this is the case even when the source of illumination is of the most powerful description.

This list of the different forms of apparatuses used in the synthesis of movement is, no doubt, incomplete; but it may serve to indicate the respective advantages and disadvantages of each system, and to serve as a guide to those who may wish to make fresh researches in the same direction.

The Points of a Good Apparatus.—In apparatuses in which the figures rotate with a continuous movement the image can only be made to appear motionless by giving such a short exposure that the movement during that time is inappreciable.

Now, the brevity of the period of illumination entails a considerable loss of light, and hence the image, when projected on a large scale, is hardly visible at all. If, on the other hand, it is necessary to produce a brilliant projection, the duration of the exposure must be as long as possible; in that case, however, the image which is for the time being under observation must be absolutely motionless. It is obviously impossible to ensure alternate periods of rest and motion with discs or other heavy pieces of revolving apparatus. The solution of this problem is the same as that which we adopted in chronophotography. The apparatus which is used for the analysis of movement is reversible, at least in principle, and might be used for their synthetic reconstruction. Let us imagine that a strip of film has imprinted on it positive images, and that this strip is placed at the focus of the object-glass, and brightly illuminated from behind. If these figures are then projected on a screen as far removed from the object-glass as were the original objects, the figures will appear to actual scale.

Every time the objective is exposed by the rotation of the diaphragm an image is thrown on the screen, the outlines of which are perfectly defined, because the film is arrested by compression at the moment of exposure. As a matter of fact, it is better to adopt a special contrivance for projecting moving figures.

The following are the reasons which induced us to construct a new instrument, to which we have given the name, "Chronophotographic projector."

The Chronophotographic Projector.—In a projecting apparatus the exposure should be as long as possible, and the transparency should be arrested during the whole period of its projection upon the screen. These conditions must be fulfilled if bright and clear images are required. In the case of the analyzing apparatus, the exposures, on the contrary, should be as brief as the illumination will allow. For an insect's wing, the exposure should be no more than 25000 part of a second. Now, with such a short exposure, an image would be almost invisible if greatly enlarged by projection; and this would still be the case even were the source of illumination very powerful. The most important point in constructing a projector is to secure as long an exposure as is possible. For instance, if ten images were taken per second, the exposure should be half or a third as long; that is to say, for 1/20 or 1/30 of a second, instead of for 1/1000 of a second, which is the usual exposure allowed by an analyzing apparatus. Instead of the small fenestrations on the circular diaphragms, long slits should be made, occupying a third of their circumference. During this long exposure, the film should be completely arrested, and for this purpose the compressor should have a particular kind of cam.

To realize the nature of a movement satisfactorily, it is as well to reproduce it several times. This may easily be done by an apparatus fitted with revolving discs; but, as in our apparatus, we have to use a length of film, it should be glued together at the ends, so as to produce an endless series of images continually rotating at the focus of the objective. Such a strip as this could not be introduced into the ordinary chronophotographic apparatus.

We have therefore constructed a special apparatus, in which an endless length of film containing forty or sixty figures, or even more, is allowed to pass without cessation under the field of the objective.

The illumination, which is from behind, and consists either of the electric light or the sun itself, projects these figures upon a screen. This instrument produces very bright images, but it is noisy, and the projected figures do not appear as absolutely motionless as one could wish.

Having arrived at this point in our researches, we learned that our mechanic had discovered an immediate solution of this problem, and by quite a different method; we shall therefore desist from our present account pending further investigations.

No comments:

Post a Comment

Comment moderation is turned on. Your message will appear after it has been reviewed.