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]
[Illustration]
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.
It is curious to see that this expression of successive attitudes of
the trunk and limbs, by means of a series of lines expressing the
direction of the bones, has been precisely adopted by the ancient
authors as being the most explicit and capable of making the phases of
a movement understood. Thus, Vincent and Goiffon, in their remarkable
work on the horse, have tried to represent by lines at different
angles the displacements of the bones of limbs while taking a step.
It is not necessary to expatiate on the superiority photography has
over actual observation for this purpose, giving the true positions of
the limbs, while the eye is incapable of taking in such rapid actions
in such short spaces of time.
At the commencement of this century the brothers Weber had recourse
to the same mode of representation to explain the successive actions
produced in the walk of a man. It was by reducing the walker to
the figure of a skeleton that these eminent observers succeeded in
presenting, without confusion, a number of images expressing different
attitudes.
The method of constructing the bright metal bands which in the
photograph explain the position of the joints, requires special
mention. As the length of exposure is very short, a substance having
great brilliancy must be employed. The strips of metal are not equally
luminous down their entire length, because they do not reflect the
solar rays at the same angle; they present lines of unequal intensity
on the negatives. I have obtained the best results with small strips of
black wood with nails having hemispherical bright metal heads driven in
at regular intervals. Each little rounded surface reflected the image
of the sun very brilliantly. In the photograph these lines of nails are
reproduced as dotted lines. At the ankle, knee, and hip joints, nails
of larger dimensions were inserted, showing these centers of movement
by a much larger dot.
Partial photographs obtained by this method allow of the different acts
of locomotion being analyzed, as well as the movements of walking,
running, or jumping.
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