CHAPTER II.

CHRONOPHOTOGRAPHY. Instantaneous photography has been of the greatest possible use to science, especially that branch of it which has been termed “chronophotography.” It is to the investigations of Mr. Muybridge and M. Marey that we are indebted for the most valuable researches on the subject. Chronophotography consists in taking a number of photographs of any object at short and regular intervals of time. This is accomplished in many ways, and results obtained are useful for many purposes. The graphic method has been of great service in almost every branch of science, and laborious statistics obtained by computation have been replaced by diagrams in which the variation of a curve expresses in the most striking manner the various phases of some patiently observed phenomena. Furthermore, by the methods of modern science, a recording apparatus has been devised which, working automatically, traces the curves of such physical or physiological events which, by reason of their slowness, feebleness, or their speed, would otherwise be inaccessible to observation. The development of these methods of analyzing movement by photography have enabled the researches of physiological laboratories to become of the greatest possible value. The matter in this chapter is very largely an abstract of M. Marey’s researches, which were originally published in “_La Nature_,” and their publication in the “Scientific American Supplement” extended over a period of several years. Subsequent to this publication M. Marey wrote a book called “_Le Mouvement_,” which has been translated by Mr. Eric Pritchard under the title of “Movement.” It is published in the International Scientific Series; and for a more extensive and scientific treatment of the subject than we are able to give here, we refer our readers to this excellent work. M. Marey describes the rudiments of chronography by supposing we take a strip of paper which is made to travel by clockwork at a uniform rate. A pen affixed above the paper marks, as it rises and falls alternately, the various periods and intervals. When the pen comes in contact with the paper it leaves a record in the form of dashes of different lengths at varying intervals. If the dashes should be equidistant it shows that the periods of contact follow one another at equal intervals of time. Now, as it is known that the speed at which the paper travels is so many inches or feet per second, it is an easy matter to obtain an accurate measurement of the duration of contact and of the intervals between. In brief, this is the principle of chronography. Chronophotography is simply an amplification of this system and has many advantages, rendering measurements possible where the moving body is inaccessible. In other words, there need be no material limit between the visible point and the sensitized plate. Mr. Muybridge’s experiments on the gaits of the horse are famous. He used a battery of cameras as shown in our first engraving. Some of the results obtained are shown in Fig. 2. [Illustration: FIG. 1.--ARRANGEMENTS ADOPTED BY MR. MUYBRIDGE IN HIS EXPERIMENTS ON THE GAITS OF A HORSE. On the left is the reflecting screen against which the animal appeared _en silhouette_. On the right is the series of photographic apparatus, of which each one took an image.] [Illustration: FIG. 2.--TWELVE SUCCESSIVE PHOTOGRAPHS, BY MR. MUYBRIDGE, OF A HORSE IN FULL GALLOP. In the last figure the horse is shown standing still. The speed of the horse was about 1,142 meters (3,746 feet) per minute.] [Illustration: FIG. 3.--CHRONOPHOTOGRAPHIC TRAJECTORY OF A BRILLIANT BALL THROWN ACROSS THE BLACK SCREEN.] [Illustration: FIG. 4.--CHRONOPHOTOGRAPHIC APPARATUS PRODUCING UPON ONE PLATE A SERIES OF PHOTOGRAPHS AT EQUAL INTERVALS OF TIME. The apparatus is open and shows the position of the disk, with its openings moving in front of the plate.] In Mr. Muybridge’s arrangement, photographic instruments faced a white screen before which passed an animal walking, trotting, or galloping. As fast as the animal advanced, the shutters of the lenses opened and permitted the taking of negatives of the animal. These were, of course, different from each other, because they were taken in succession. They therefore showed the animal in the various attitudes he assumed at different instants during his passage across the field covered by the instruments. The dazzling white light brought out _en silhouette_ the body of the animal. Each shutter is actuated by a powerful spring; the shutter is opened as the animal advances. Threads may be observed across the road; the animal, breaking these threads one after the other, opens the shutters. Mr. Muybridge varied his experiments most successfully. He studied the gaits of different animals, and those of men in jumping, vaulting, and in the handling of various utensils. But since this time the progress of photographic chemistry has wonderfully increased the sensibility of the plates, and at the present day more than mere silhouettes of moving animals and men can be obtained. In a good light full images with all desired relief can be obtained. For example, if an athlete in motion is photographed, all of the muscles of the body are perfectly traced in relief, indicating the parts taken by each of them in the movement executed. The methods used by Mr. Muybridge would always suffice to illustrate the successive phases of the displacement of the members if they were taken at equal intervals of time, but the arrangements adapted for bringing about the formation of the successive phases cause irregularity in the extent of these intervals. The threads give more or less before breaking; moreover, the progress of the horse is not at an even rate of speed. Nevertheless, Mr. Muybridge endeavored to develop from a series of images the trajectory of each leg of a horse, but the curves obtained in these laborious attempts had not sufficient precision. A very simple method enables us to obtain, with perfect fidelity, the trajectory of a body in movement; it is the photographing of this body in front of a black surface. If the photographic apparatus is directed against a black screen, the objective can be uncovered without effect on a sensitized plate, as it will receive no light; but if a white ball strongly illuminated by the sun is thrown across the plane of this screen, and parallel with it, its image will be reproduced upon the plate, which will show the track of the ball in its trajectory, just as the eye receives a momentary impression of lines of fire when a lighted piece of charcoal is waved through the air at night. [Illustration: FIG. 5.--GENERAL VIEW OF THE PHYSIOLOGICAL STATION AT PARIS.] [Illustration: FIG. 6.--DARK CHAMBER ON WHEELS.] [Illustration: FIG. 7.--INTERIOR ARRANGEMENT OF THE DARK CHAMBER.] Fig. 3 shows the parabolic trajectory of a brilliant ball thrown across the face of a dark screen; but it is discontinuous, as exposures were only produced each fiftieth of a second on account of the number of the openings and the speed of the rotation of the disk. This is only an example which shows the almost limitless number of varieties of movement which may be analyzed by chronophotography. With ordinary shutters it would be difficult to obtain this quickness, but the perforated disk which is used in chronophotography gradually acquires a speed of rotation that may be very great. Fig. 4 shows the arrangement of this disk by which a rotary movement is imparted by a powerful gearing controlled by a regulator. As soon as the disk obtains a speed of ten turns a second, the regulator maintains this speed with perfect uniformity. The disk moves in front of the sensitized plate a few millimeters only; then, knowing the angular value of each of the openings, the period of exposure is easily deduced therefrom. [Illustration: FIG. 8.--WALKING MAN, CLOTHED IN WHITE, PASSING ACROSS THE FIELD.] The condition most difficult of fulfillment is the absolute darkness of the screen before which the photographs are taken. Little light as there is, the screen might reflect upon this sensitized plate, during a single exposure, small quantities of light, which would tend to fog the plate. A wall painted with any black pigment, or even covered with black velvet, exposed to the sun, reflects too much light for a plate to withstand. The term “black screen” is used in a metaphorical sense. In reality the work is done before a dark cavity, being in truth what is known as “Chevreul’s black.” To obtain these favorable conditions, a chamber nearly thirty-three feet deep and of equal breadth was constructed; one face of this chamber was open, and restricted by movable frames to the exact height necessary. The interior of the chamber was completely blackened, the ground was coated with pitch, and the back hung with black velvet. [Illustration: FIG. 9.--INSTANTANEOUS PHOTOGRAPH OF A MAN JUMPING OVER AN OBSTACLE.] Before entering into a detail of the experiments, we shall point out the general arrangement of the Physiological Station of Paris. Fig. 5 gives a general view of the grounds and buildings. On these grounds, which were laid out by the city of Paris as a nursery, there is a circular road, thirteen feet wide, designed for the exercise of horses, and, outside of this, a footpath for men. All around this road there runs a telegraph line whose poles are spaced 164 feet apart. Every time that a person walks in front of a pole a telegraphic signal is given, and this is inscribed in one of the rooms of the principal building. Further on we shall speak of this sort of automatic inscription, by means of which we ascertain at every instant the speed of the walker, the variations therein, and even the frequency of his steps. In the center of the track there is a high post that carries a mechanical drum which regulates the rhythm of the gait, and which is actuated by a special telegraph line running from one of the rooms in the large building, wherein the rhythm is regulated by a mechanical interrupter. From the center of the circle, likewise, there starts a small railway upon which runs a car that forms a photographic chamber, from the interior of which is taken a series of instantaneous images of the horses or men whose gait we desire to analyze. Fig. 6 represents the photographic chamber in which the experimenter places himself. This chamber is mounted upon wheels, and runs upon a railway in such a way that it can approach or move away from the screen according to the objectives that are being used and to the size of the images that it is desired to obtain. As a general thing, it is advantageous to place the photographic apparatus quite far from the screen, say about 164 feet. From this distance the angle at which the subject whose image is being taken does not change much during the time it takes to pass before the black screen. From the exterior of this chamber are seen the red windows through which the operator can follow the different motions that he is studying. To have the different acts performed he gives his orders through a speaking trumpet. The front of the chamber is removed in Fig. 6 in order to show a revolving disk provided with a small window through which the light enters the photographic objective intermittently. This disk is of large dimensions (four and three-quarters feet in diameter), and the window in it represents only one hundredth of its circumference. It follows from this that if the disk makes ten revolutions per second, the duration of lighting will be but _one thousandth of a second_. Motion is communicated to the disk by a train of wheels which is wound up with a winch and which is actuated by a weight of one hundred and fifty kilograms placed behind the chamber. The motion of the disk is arrested by a brake, and a bell maneuvered from the interior serves to give orders to an aid either to set the disk in operation or to stop it. Fig. 7 shows the inner arrangement of the chamber, a portion of one of the sides being removed to show the photographic apparatus, A, placed upon a bracket before the screen. This apparatus receives long and narrow sensitized plates that exactly hold an entire image of the screen. At B is the revolving disk which produces the intermittent illuminations, and at D is a cut-off which is raised vertically at the beginning of the experiment, and which is allowed to fall at the end so as to allow light to enter only during the time that is strictly necessary. E is a wide slit in front of the objective, for allowing the latter to take in the field in which are occurring the motions that are being studied. The darkness that reigns in the rolling chamber permits of manipulating the sensitized plates therein at ease, and of changing them at every new experiment. [Illustration: FIG. 10.--INSTANTANEOUS PHOTOGRAPH OF A MAN WALKING.] [Illustration: FIG. 11.--MAN CLOTHED IN BLACK VELVET. The axes of the limbs are traced by white cords; the joints carry white buttons placed at the point of rotation. The head is covered by a helmet of black velvet which completely hides it, and to which is affixed a bright ball at the level of the ear.] [Illustration: FIG. 12.--CHRONOPHOTOGRAPHIC IMAGES OF A RUNNER. Below the figure is a scale whose divisions are 0.50 meter (19-7/10 inches) long, and serve to give the extent of the movements.] Against the dark field just described, a man placed in full light, naked, or clothed in white, gives a sharp image on the sensitized plate. The results in running and jumping which are obtained by this means are very satisfactory. For scientific purposes it is found that the results are better if, instead of white clothing, the runner is clothed in black velvet. By this means he becomes nearly invisible before the black area. If white cords are attached to this costume, following the direction of the axes of his limbs, and white buttons used for the principal articulations, the white parts are reproduced and reobtained on the sensitized plate in an almost unlimited number of positions. [Illustration: FIG. 13.--OSCILLATIONS OF THE LEG OF A WALKING MAN.] [Illustration: FIG. 14.--SUCCESSIVE POSITIONS OF THE LIMBS IN AN ELASTIC JUMP UPON THE BALL OF THE FOOT.] Using a disk pierced with five holes, which gives twenty-five images per second, the result shown in Fig. 12, which shows in full detail the movements of the left half of the body, head, arm, and leg, was obtained by this method for the action of running. Every fifth image is a little stronger than the others. This is effected by making one of the apertures in the disk larger than the others. The time of exposure is thus increased, and the intensity of the image is greater. The object of this disposition is to furnish base marks, by means of which it is always easy to recognize traces corresponding to the same image, that is to say, to a given attitude of the runner. For detailed studies a part of the image is screened, as shown in Fig. 13. These diagrams are very well adapted for the comparison of two sorts of movements whose difference cannot be discerned by the eye. Thus, in jumping from an elevation the shock caused by the feet striking the ground is reduced in intensity by bending the legs, while the extensor muscles operate to sustain the weight of the falling body. Our next two engravings show two kinds of jumps: the first, the flexure of the legs and the reduction of the shock; the second, with the leg almost straight, which implies a severe shock by the feet striking the ground. [Illustration: FIG. 15.--INELASTIC JUMP UPON THE HEELS.] The practical applications of chronophotography are soon seen. Just as machines are driven so as to obtain a useful effect at the smallest expenditure of power, so a man can govern his movements so as to produce the wished-for effects with the least waste of energy, and, consequently, with the least possible fatigue. Of two gaits which can carry us over a definite space in a given time, the one should be preferred which costs the least possible fatigue. Chronophotography furnishes the missing elements of the problem, giving exactly the velocity of the different parts of the body, by the balancing of which we can determine the masses in movement. From a long series of comparisons, important conclusions can be drawn, as, for example, the following: in walking, the most favorable gait is one where step succeeds step at the rate of about one hundred and twenty a minute; for running, the step should be nearly two hundred and forty a minute. Fewer or more numerous steps will give less effect at a greater expenditure of the work. The applications are therefore obvious; they enable us to fix the rate of steps of soldiers to economize as much as possible their strength in the severe trials to which they are subjected. These studies have been followed out at great length, under varying conditions, using a considerable number of subjects; and the results, while not final, have shown that the true method has been found. Experiments have confirmed that which the laws of mechanics could not foretell when the dynamic conditions of the work of man were incompletely known. [Illustration: FIG. 16.--OSCILLATION OF THE FORE LEG IN A GALLOP. INTERVAL BETWEEN EXPOSURES ONE TWENTY-FIFTH OF A SECOND.] M. Marey’s studies of the legs of the horse are particularly interesting. We give one engraving showing the oscillation of the fore leg of a horse in a gallop. The analysis of the flight of birds presents special difficulty. Owing to the extreme rapidity of the movements of the wings, an extremely short exposure is required. The direction, often capricious, of the flight of the bird, and the length of the path which must be followed, to include on the sensitized plate sufficiently sharp images, add to the difficulty. Several repetitions of the same experiment are generally required before success. The photographic gun is particularly valuable for taking photographs of birds. Our engravings show the mechanism of the photographic gun and the method of using it. We present a photograph of a gull taken during its flight and an enlargement of the same. The photographic gun will be understood by reference to the engraving, and is fully described in the “Scientific American Supplement,” No. 386, to which the reader is referred. We also give photographs of a pigeon rising in flight and the successive attitudes of a gull. Space forbids us to more than state that the analysis of the flight of birds is a most interesting and important subject, and the results obtained by chronophotography are most gratifying. [Illustration: FIG. 17.--MODE OF USING THE PHOTOGRAPHIC GUN.] [Illustration: FIG. 18.--MECHANISM OF THE PHOTOGRAPHIC GUN. 1.--General View of the Apparatus. 2.--The Shutter and Perforated Disk. 3.--Box containing Twenty-five Sensitized Plates.] [Illustration: FIG. 19.--PHOTOGRAPH OF A GULL TAKEN DURING ITS FLIGHT.] [Illustration: FIG. 20.--ENLARGEMENT OF AN IMAGE TAKEN BY THE PHOTOGRAPHIC GUN.] [Illustration: FIG. 21.--ENLARGEMENT OF ANOTHER IMAGE OF A BIRD TAKEN BY THE SAME APPARATUS.] The analysis of locomotion in water is one of the most interesting developments of chronophotography. In order to study locomotion in water it was necessary to modify the method. The animals experimented with swam in a glass-sided aquarium fitted in an aperture in a wall, as shown in our engraving. The aquarium was directly illuminated by the light of the horizon, forming a very clear field upon which the animals were outlined as silhouettes. Sometimes the external glass of the aquarium was covered by letting down an opaque shutter; then, upon opening another shutter, placed above the water, the brightly illuminated animals were seen standing out from the black field. In most cases it was found necessary to operate before the luminous ground, so it was not possible to receive several successive images upon a removable plate, but it was necessary to cause the sensitized surface to move by starts, so as to bring before the objective points which were always new for each new image that is to be formed. A flexible gelatino-bromide-of-silver film was used. The film was cut into a long and narrow strip which in the camera passed along at the focus of the objective, and unwound from a supply bobbin, and wound around a receiving one. [Illustration: FIG. 23.--PIGEON RISING IN FLIGHT. The successive images correspond to less and less advanced phases of the wing’s revolution.] [Illustration: FIG. 23.--ELEVEN SUCCESSIVE ATTITUDES OF A FLYING GULL. In this series of images, traced from the originals, the distances representing the positions of the bird in space are exaggerated to avoid confusion.] The objective turned toward the right has a slit in the center for the passage of the diaphragm which, in revolving, allows the light to pass intermittingly. When the small diaphragm makes one revolution the large one makes five revolutions, and it is then only that the apertures meet and the light passes. The bellows behind the objective allows the light to reach the sensitized film. The box is, of course, tightly closed. The focusing is done by means of a small telescope or spy glass. It is necessary at each new experiment to use a new band of film, and the substitution of rolls of films is effected in the light by means of bobbins upon which the film is rolled. At the extremity of each band of film are glued paper bands of the same width. One of these prolongations is red and the other is black. Each of them is about twenty inches in length. Having the two colors makes it almost impossible to reëxpose a film, as one is not liable to confound a bobbin which has been used with one that has not, the color of the roll being different. Special devices are employed in the camera to render the film immovable for an instant while it receives the impression from the object. Arrangements are also provided for obtaining a uniform velocity. The use of the apparatus which we have just described permitted of seeing with what a variety of means of locomotion the various kinds of aquatic animals--fishes, mollusks, crustaceans, etc.--propel themselves. The motion of the medusa is particularly interesting, and the phases of the movement of the umbrella are shown in Fig. 26. The propulsion of this mollusk is effected through the alternate contraction and dilation of its umbrella. Ten images per second were sufficient to obtain a pretty complete series of the phases of this motion. These images gain much by being examined in the zoetrope, wherein they reproduce with absolute perfection the aspect of the animal in motion. [Illustration: FIG. 24.--ARRANGEMENT OF THE AQUARIUM FOR THE STUDY OF AQUATIC LOCOMOTION.] The hippocampus, which is otherwise known as the “sea-horse,” affords another interesting example of aquatic locomotion. The principal propeller of this animal is a dorsal fin which vibrates with such rapidity that it is almost invisible, and has an appearance analogous to that of the branches of a tuning fork in motion. With twenty images per second it is seen that this vibration is undulatory. We have before us the successive deviations of the lower, middle, and upper rays of the film. In the present case the undulation takes place from the bottom upwards. [Illustration: FIG. 25.--PHOTOCHRONOGRAPHIC APPARATUS.] The comatula is habitually fixed to the bottom of the aquarium, just as a plant is fixed to the earth by its roots. It therefore makes nothing but vague motions of the arm, which it rolls up and unrolls; but if the animal be excited by the means of a rod, it will be observed to begin a strange motion which carries it quite a distance. In this kind of locomotion the ten arms move alternately; five of them rise and keep tightly pressed against the calyx, and the other five descend and separate from it. Upon the arms that rise, the cirri are invisible, and while upon those that descend, they diverge in order to obtain a purchase upon the water. These motions of the cirri seem passive, like those of a valve that obeys the thrust of a liquid. M. Marey says: “I have obtained images of a certain number of other aquatic species, the swimming of the eel, the skate, etc. These types of locomotion ought to be studied methodically, compared with each other, and considered in their relations with the conformation of the different species. It will, I hope, be a new element for the interpretation of the laws of animal morphology, which are very obscure.” [Illustration: FIG. 26.--MOTIONS OF THE UMBRELLA OF THE MEDUSA.] M. Marey has also investigated the flight of insects by means of chronophotography. These experiments are most delicate and interesting, and the results obtained go a long way towards making up a satisfactory theory of insect life. M. Marey says that the wing in its to-and-fro movements is bent in various directions by the resistance of the air. Its action is always that of an inclined plane striking against the fluid, and utilizing that part of the resistance which is favorable to its onward progression. This mechanism is the same as that of a waterman’s scull (reference of course being to “sea sculling”, and not to “river sculling”), which, as it moves backward and forward, is obliquely inclined in opposite directions, each time communicating an impulse to the boat. There is, however, a difference between these two methods of propulsion. The scull used by the waterman offers a rigid resistance to the water, and the operator has to impart alternate rotary movements to the scull by his hand--at the same time taking care that the scull strikes the water at a favorable slant. The mechanism in the case of the insect’s wing is far simpler. The flexible membrane which constitutes the anterior part of the wing presents a rigid border which enables the wing to incline itself at the most favorable angle. The muscles only maintain a to-and-fro movement. The resistance to the air does the rest, namely, effects those changes in surface obliquity which determine the formation of an 8-shaped trajectory by the extremity of the wing. M. Marey states that he succeeded in obtaining a photograph of the gilded wing of an insect, which, though not absolutely at liberty, could fly at a comparatively high rate of speed. The photographs of the trajectory of the wing of an insect are very interesting. A wooden box was lined throughout with black velvet. The bottom of the box, a simple disk supported by a foot piece, was placed in position; the periphery of the space was covered with a white material, leaving between it and the central disk an annular track covered with black velvet. It was around this annular track the insect was made to fly. A needle stuck in the middle of the disk served as an axis for a revolving beam and its counterbalance. This beam consisted of a straw, and at the end of it was fixed a light pair of forceps to hold the insect. The dragon fly commenced flying around the track at a very rapid rate, drawing the straw after it. The gold spangles passing through his wings described a trajectory which was easily photographed. [Illustration: FIG. 27.--MOTIONS OF THE DORSAL FIN OF THE SEA-HORSE.] The chronophotography of insects by the use of a moving film has been also accomplished by means of very ingenious apparatus. In some cases the insects were held in forceps, and in other cases they were allowed free flight in a cardboard box. “Comparative locomotion,” which is rendered possible by chronophotography, might almost be called a new science. It is, at any rate, an important adjunct to the studies of the zoölogist. The researches of M. Marey upon the different terrestrial mammals, birds, tortoises, lizards, frogs, toads, tadpoles, snails, eels, fish, insects, and arachnids are of the greatest possible value and interest. The applications of chronophotography to experimental physiology are numerous. It supplements the information obtained by the graphic methods. It has rendered possible the photography of the successive phases of cardiac action in a tortoise under condition of artificial circulation. The mechanism of cardiac pulsation has also been studied by its means, as well as the determination of the centers of movements in joints. It has been found that chronophotography could be applied not only to objects of considerable size, but to those of microscopic size as well. Special arrangements of apparatus are necessary for this purpose. By its means the retraction of the spiral stalks in vorticellæ, the movement of the blood in capillary vessels, and the movements of the zoöspores in the cells of conferva have been determined. [Illustration: FIG. 28.--MOTIONS OF THE COMATULA.] [Illustration: FIG. 29. TORPEDO BADLY FIRED. TORPEDO PROPERLY FIRED.] The great value of chronophotography is unquestionable for use in every case where the body whose rapid changes of position or form we wish to know is inaccessible to us, or its movements cannot be mechanically traced. Chronophotography has been used in France for studies touching the military art, being employed for registering the firing of projectiles having a relatively slow motion, such as the explosion of stationary torpedoes, the recoil of guns, the motion of automobile torpedoes, etc. Special arrangements are provided to permit of electrically controlling the phenomenon to be photographed. The apparatus is described in detail in the “Scientific American Supplement,” No. 743. We present a diagram showing the results obtained by photographing the firing of torpedoes. Although the velocity of these projectiles is not very great, about sixty feet per second, it is yet very difficult for the eye to take exact account of what is occurring during the launching. As the net cost of a torpedo is considerable, it is essential that the conditions which influence the regularity of its submarine flight shall be known with precision. If it inclines in front more or less in plunging, the regularity of its running will be put to hazard; if, on the contrary, it falls flat upon the water, the results will be very different. Our engraving shows the torpedo starting from the tube and traversing the different panels in the field of firing. In the first half the torpedo, gradually inclining, falls point foremost; it has been badly fired. In the second series, on the contrary, the torpedo is maintaining itself horizontally, and, in a manner, moving always parallel with itself. Under such circumstances it falls flat and starts off normally and regularly to the object to be reached. This shows the great utility of chronophotography. AN AMATEUR CHRONOPHOTOGRAPHIC APPARATUS. [Illustration: FIG. 1.--AMATEUR’S CHRONOPHOTOGRAPHIC APPARATUS.] The experiments which we have been describing necessitate apparatus of the most expensive kind, and they are unadapted for the use of the amateur. The apparatus of M. Georges Demeny, which we illustrate, is, however, very simple. The reader needs to be reminded that there are three types of chronophotographic machinery in use, in two of which a single objective, with a disk shutter revolving at great speed, is employed. In one the object shifts, and gives several images from an immovable plate, while in the other the object is stationary, and the movable sensitized surface gives well-separated images. The third method, which is the least interesting, consists in taking as many objectives and plates as it is desired to have images, and in freeing the shutters of each objective, one after the other. The most scientific solution of the problem is that which permits of obtaining upon a band of film, and with a single objective, a succession of well-separated images whose number depends only upon the length of the band employed. The difficulty in using a sensitized band consists in arresting it for the very brief instant in which each image impresses the plate. The Demeny apparatus which we are about to describe is very simple. A wooden box having about the dimensions of an ordinary seven by nine inch apparatus is provided with an objective of wide aperture, of which only the center is utilized. Back of this objective, and as near as possible to the sensitized surface, the disk shutter is revolved by means of a crank. Up to this point there is really nothing new in the apparatus; but the principal improvement consists in the unwinding and arrest of the sensitized film. Number 1 of our first engraving represents the principle of the system. Two disks, R and P, are each mounted upon an axis passing through their centers; bobbins that carry the films are fixed, one of them at R, upon a spindle mounted in the axis of rotation of the disk, and the other at P, upon a spindle mounted eccentrically to such axis. It is this eccentric position that chiefly constitutes the invention. Let us suppose that the two bobbins are in place, as shown in cut. The film wound upon A, having one of its extremities attached at B, follows the course, C, S, during which it passes behind the objective; the two bobbins cannot have any proper motion in consequence of the method of fixing which is adopted; they and the disks, R and P, that support them, become interdependent. Because the disk, P, revolves, the film coming from A will wind around B; but, in consequence of the eccentric position of this bobbin upon the disk, traction will cease to occur for a very brief instant at the moment at which the bobbin, B, approaches A as closely as possible, Despite this, as the winding always proceeds to a degree proportional to the unwinding, the film remains perfectly taut. It is at this moment that the window, H, of the disk, L, uncovers the objective for an instant. It will be understood that the crank, M, sets the disk in motion, and it is this, through a mechanism of gears, that controls the operation of the bobbins. There is, therefore, an exact mathematical coincidence between the arrest of the film and the passage of the window, and this is essential for the sharpness of the image. This would not always occur if a friction device was depended upon for the rest of the film, for in this case a sliding might occur which would produce a blurring of the image. The solution offered by the Demeny apparatus is, therefore, the simplest and one of the surest known. The simplification of the mechanism has permitted of constructing an apparatus light enough to allow of operating without a tripod, by holding it in the arms, as shown in our second engraving. Each film terminates in a strip of black paper glued to it, and forms a complete covering after the winding upon the bobbin. This arrangement protects the sensitized part from the light, and permits of changing the bobbin in daylight. Twenty of them can be stored in the spaces in the box left by the mechanism, so that one may always have a large supply on hand. [Illustration: FIG. 2--METHOD OF USING THE DEMENY APPARATUS.]