PHOTOGRAPHS FOR STUDYING THE MOVEMENTS OF MEN AND ANIMALS.
By M. Marey.[2]
When a series of photographs representing the successive attitudes of an animal is taken on the same plate, it is naturally desirable to multiply these images, for the purpose of getting the greatest possible number of phases of the movement. But when the animals to be reproduced do not move rapidly, the number of images is limited by their superposition and the resulting confusion. Thus, a man running at a moderate pace may be photographed ten times in a second, without the impressions on the plate being confused. If, at times, one leg is depicted on a part already bearing the trace of another leg, the superposition does not alter the image; the whites become only more intense in those portions of the plates receiving an impression twice over, but the contours of both limbs are still to be distinguished. In the case, however, of a man walking slowly, these superpositions are so numerous as to render the reproduction very confused.
It is to remedy this defect that I have had recourse to partial photography; that is to say, I have suppressed certain parts of the image, that the rest may be more easily understood.
In the method which I employ, only white and light objects affect the sensitive plate; it suffices, therefore, to clothe that portion of the body to be suppressed in black. If a man dressed in a parti-colored costume of black and white walk over the track, by turning the white parts of his apparel toward the camera—the right side, for instance—he will be reproduced as if he only possessed the right half of his body. These images permit the various successive phases of movement to be accurately followed, the rotation of the foot and leg when both on the ground and lifted up, and the oscillation of the limb at the hip joint while moving along in a continuous manner.
These partial photographs are also useful in the analysis of rapid movements, because they allow of the number of attitudes represented being multiplied. At the same time, as a man's leg is rather large, its reproduction cannot be multiplied very often, owing to confusion by superposition. I have therefore sought to diminish the size of the images, so as to an admit of repetition at very short intervals. The method consists in attiring a walker in a black costume having narrow bands of bright metal applied down the length of the leg, thigh, and arm, following exactly the direction of the bones of the limbs. This plan permits the number of images formerly produced to be increased at least tenfold; thus, instead of ten photographs per second, one hundred may be taken. To do this it is not necessary to change the speed of rotation of the disk, but instead of piercing it with one aperture, ten holes are made equally disposed around the circumference.[3]
The figure here shown is from one of the negatives projected on the screen from the lantern. The dotted lines have been filled in to form direct lines. The figure shows the successive phases of one step in running. Only the left leg is represented; the lines correspond to the thigh, leg, and foot; the dots to the joints at the ankle, knee, and hip.
This diagram shows pretty clearly the alterations of flexion and extension of the leg on the thigh, the undulating trajectories of the foot, knee, and hip, and yet the number of images does not exceed sixty in a second. A revolving shutter pierced with more holes would give more perfectly the angular displacements of the leg on the thigh, and the positions of the three joints. The finer the dotted lines expressing the direction of the limbs, the more the images may be multiplied; but in the present case, sixty times in a second more than suffice to show the displacements of the limbs when running.
In this photographic analysis the two factors of movements—time and space—cannot be both estimated perfectly; knowledge of the positions the body has occupied in space requires that one should possess complete and distinct images; in order to obtain such images, a sufficiently long space of time must elapse between the two successive photographs. If, on the contrary, it is desirable to estimate time more perfectly, the frequency of recurrence of the image must be greatly increased. To bring these two exigencies as closely together as possible, lines and points must be chosen for the partial photographs which best show the successive attitudes of the body.