Sunday, October 26, 2014
Chronophotography, Part 2 -- October 26, 2014
An excerpt from "Chronophotography" from Magic: Stage Illusions and Scientific Diversions, Including Trick Photography, edited by Albert Allis Hopkins. We learned about Eadweard Muybridge in Part One:
In Part Three we will read about the analysis of locomotion in water.
Be sure to click on the images to see larger versions.
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.
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.
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.
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 you 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.
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.
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.
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.
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.