ABERRATIONS OF ANIMATED PHOTOGRAPHY
It has been pointed out in a previous chapter that cinematography
is nothing more or less than an optical illusion. Further proof of
this assertion exists in plenty. When following the projection of a
picture upon the screen, one is often perplexed by a curious effect,
or a movement which appears to be in opposition to all the known laws
of motion. This happens not only in trick work where such odd and
startling effects are introduced purposely, but in straightforward
every-day topical subjects.
For instance, it must have been noticed that when a ship or railway
train is in rapid movement, and is photographed from a fixed stationary
point, such as the quay or platform, the moving object appears to
stand out in bold relief against the background. One gathers a very
comprehensive idea of its length, width, height, and the comparative
size of all its integral parts, such as the guns on the ship's deck or
the locomotive's cylinders. It is a curious stereoscopic effect, but
at the same time is not truly so, because it is only the moving object
which appears to possess solidity. The foreground and background
remain as plane surfaces so that it is impossible to obtain an idea of
distance. This effect arises from the fact that what might be described
as the central part of the picture is moving or continually changing,
thereby compelling all the objects attached to its length and breadth
to assume relief in regard to the other parts of the picture.
But if the camera with which the pictures are taken is placed upon the
moving object itself, then the whole of the resulting picture stands
out in a truly stereoscopic manner. One gathers an impression of
distance between the various objects on the screen. Everything is shown
with form and solidity in precisely the same way as if one were looking
through a hand stereoscope upon a photograph taken stereoscopically.
This effect is due to the fact that all the planes are moving
continually.
But probably the most bewildering puzzle is the moving wheel. A
carriage or waggon is seen advancing across the screen from left to
right, but the spokes of the wheels, on the other hand, seem to be
moving in the opposite direction. At other times the spokes move in
successive spasmodic jumps, or appear to be stationary, so that a
curious skidding effect is produced, notwithstanding that the rim
itself is seen to be revolving normally.
There have been many explanations of this extraordinary effect, and in
one instance the higher mathematics were pressed into service without
any great success. The most convincing explanation known to the writer
is that given him by Monsieur Lucien Bull, the assistant-director
of the Marey Institute, where phenomena of this class are minutely
investigated, because they accord with the work of that unique and
admirable institution. By Monsieur Bull the illusion was explained very
easily, but, curiously enough, in carrying out the experiments to this
end, he encountered another illusion equally strange.
[Illustration: Fig. 3.--The first picture of the four-spoke
wheel.]
[Illustration: Fig. 4.--During the eclipse of the lens the
spokes have moved a distance equal to the angle between them, causing
the spokes apparently to stand still while the wheel is moving.]
To reduce the explanation to its simplest form we will suppose that a
wheel has four spokes spaced equidistantly, that is, 90 degrees apart,
and that the wheel is moving from right to left. As a matter of fact
such an example is not the best for the purpose, but it shall be taken
merely because it is the simplest to understand. An exposure is made,
the wheel being photographed in the position shown in Fig. 3. The lens
is eclipsed by the shutter, and the film is jerked downwards into
position in the gate so as to bring a fresh unexposed surface before
the lens. While this operation is taking place, we will suppose that
the wheel, continuing its forward movement, completes one quarter of a
revolution. Consequently when the second exposure is made spoke 1 has
moved 90 degrees, which is the angle between each spoke. Accordingly
it now occupies exactly the same position as that of spoke 2 at the
time of the first exposure. Spoke 2 has moved to the position formerly
occupied by spoke 3. Spoke 3 has travelled sufficiently to take the
place of spoke 4, while 4 has gone to that of 1 (Fig. 4). If four
exposures are made, and the spokes move 90 degrees each time the lens
is closed, when the four pictures are thrown successively upon the
screen they will look exactly alike. The spokes will appear to be
quite stationary, although the rim of the wheel will have moved a
distance equal to its circumference across the screen. Consequently,
if a dozen, a hundred, or a thousand exposures are made under these
conditions, the spokes moving 90 degrees between each exposure, a
quaint skidding effect will be produced. All the spokes being alike the
eye is unable to detect that any displacement has taken place between
one exposure and another. This impression of the spokes standing still
while the wheel is moving, must arise in every case in which the wheel
moves sufficiently to cause the spokes to cover a distance equal to
the angle between them during the interval while the lens is eclipsed
by the shutter. It will happen equally whether the wheel has four,
sixteen, or more spokes.
[Illustration: Fig. 5.--During the eclipse of the lens the spokes move
less than the angle (AB) between them, producing apparent backward
motion of the spokes while the wheel is running forwards.]
[Illustration: Fig. 6.--During the eclipse of the lens the spokes move
more than the angle (AB) between them, and accordingly the wheel is
seen to be moving naturally.]
Now we will suppose that the revolving speed of the wheel is retarded,
causing less than a quarter of a revolution to be completed between
each exposure. The spokes, let us say, move through an angle of 85
degrees instead of 90 degrees while the lens is eclipsed. The eye
at first receives the impression shown in Fig. 3. As the wheel only
covers 85 degrees during the eclipse, in the second picture the eye
observes that movement has occurred. Spoke 1 is now behind the point
formerly occupied by spoke 2 (shown by the dotted line in Fig. 5) in
the first exposure. The lens is eclipsed once more, and the spoke moves
another 85 degrees. When the next picture is seen spoke 1 has fallen
still farther behind the 90 degrees mark, and this indication of less
movement than the right angle becomes accentuated with each succeeding
exposure. Accordingly, the spokes in the successive pictures appear
to be moving at a less speed than the rim of the wheel, and forthwith
the eye imagines that the spokes are travelling backwards, although
meantime the wheel rim is seen to be advancing across the screen. This
remarkable effect is produced whenever the advance of the wheel is such
as to cause the spokes to move less than the angle between them, no
matter what the size of the angle may be.
We will now suppose that the revolving speed of the wheel is
accelerated so as to cause more than a quarter of a revolution to
be made while the lens is eclipsed--that the spokes move forward 95
degrees between each exposure. In this case, while the first picture
will show the position indicated in Fig. 3, the next exposure will show
spoke 1 in the position shown in Fig. 6, that is, in advance of the
angle of 90 degrees and in advance of the position occupied by spoke
2--(see the dotted line)--in the first exposure. In the third picture
the spoke will be shown still farther in advance of the right angle
mark, and the effect will be produced of the spokes apparently gaining
upon one another. When a series of pictures taken under such conditions
is thrown upon the screen in rapid succession, the spokes and rim will
be seen to be moving harmoniously in the forward and correct direction.
Accordingly natural movement of the wheel only can be shown when the
spokes of the wheel, irrespective of their number, move a distance
equal to more than the angle between them.
[Illustration: Fig. 7.--When the spokes move slightly more (AC) or
slightly less (AD) than half the angle (AB) between them, during the
eclipse of the lens, the curious illusion of seeing twice the number of
spokes in the wheel is produced.]
In the course of elucidating this problem Monsieur Bull discovered
another curious optical illusion produced by the moving wheel. Still
taking the four-spoke wheel as an illustration, we will suppose that
between each exposure the spokes are displaced a little more or a
little less than half the angle between them. As the spokes are set 90
degrees apart, the half-way point will be 45 degrees. When a succession
of such pictures is thrown upon the screen, it is not four spokes which
are seen, but eight (Fig. 7). Monsieur Bull is engaged upon a series
of experiments to ascertain why this peculiar optical illusion should
prevail, and the explanation will prove interesting.
Another interesting and more conclusive illustration of the optically
illusory properties of the cinematograph was demonstrated to me by
Monsieur Bull. In order to be absolutely positive that an apparatus
which he uses in certain cinematographic investigations should maintain
the speed he desires, he has contrived a tuning-fork control for his
electric motor. This tuning-fork resembles a large trembler blade, such
as is used in the high-tension accumulator and coil ignition system
upon motor cars. This particular instrument is timed to make, say,
40 vibrations per second, and at this speed, of course, it emits a
distinctive musical note. This tuning-fork controls the electric motor
driving the apparatus. For the purpose of illustration we will suppose
it to be necessary that the speed of the motor shall not exceed 40
revolutions per second. In the earliest experiments he depended upon
his ear to detect whether the motor and tuning-fork were in synchrony.
He varied the speed of the motor until its hum was dead in tune with
that of the tuning-fork.
But, as he thought that his ear might not be infallible, he devised
an ingenious synchronising apparatus based upon the cinematographic
principle. A small disk of cardboard provided with two holes near
its edge, at opposite points of the circumference, is mounted upon
the spindle of the tiny motor. Behind this disk is placed a small
adjustable mirror. A pencil of electric light is projected horizontally
in such a manner that it strikes the cardboard disk at right angles,
and, when a hole on the disk is brought into line with it, it passes
through and falls upon the mirror. The mirror is then set so as to
reflect and focus the pencil of light in a small circle upon the free
vibrating extremity of the tuning-fork. Naturally a strong shadow is
thrown by the latter upon the white wall behind.
In the daylight the vibration of this fork is distinctly visible, and
although it is slight and rapid it can be followed without any effort.
But when the room is darkened, the ray of light is thrown upon the
tuning-fork from the mirror. When the motor bearing the cardboard disk
is set in motion a very curious effect is produced. The pencil of light
reflected against the tuning-fork becomes interrupted twice in every
revolution of the disk, that is 80 times per second, so that, looking
at the background upon which the tuning-fork is silhouetted, the effect
produced is precisely similar to that observable upon the cinematograph
screen, where the passage of the light from the lantern is interrupted
by the rotary action of the shutter. If the revolving speed of the
motor, that is the number of revolutions per second, is the same as the
number of vibrations per second of the tuning-fork, viz. 40, the end of
the fork, as one looks at the illuminated circle on the wall against
which the shadow is thrown, appears to be at rest. One only needs to
touch the end of the fork, however, to be certain that it is vibrating.
Now if the motor be thrown out of synchrony with the tuning-fork, even
if it makes only 39 or 41 instead of 40 revolutions per second, the
disturbance is shown instantly, because looking at the illuminated
tuning-fork one observes it jumping spasmodically. This movement
becomes more pronounced as the harmony between the revolutions of the
motor and the fork is disturbed, the jumps of the blade at times being
apparently of a very severe character. Moreover, curiously enough,
under the illumination of the ray of light the erratic movements of the
blade appear to be three or four times more severe than they really
are. But as the motor revolutions and the tuning-fork vibrations are
brought into synchrony, the movements grow quieter, until at last the
tuning-fork once more appears to be quiescent.
The explanation of this quasi-cinematographic illusion, which is as
interesting and as puzzling as that of the wheel, is very simple, for
it is based indeed upon the same phenomena. As the cardboard disk is
provided with two small holes spaced 180 degrees apart, the passage
of the ray of light is intercepted by the opaque section of the disk
80 times per second when the motor revolutions and the tuning-fork
vibrations are in absolute synchrony. The result is that at this speed
the light strikes the tuning-fork each time at the instant it is at the
half-way point in its oscillating travel. One hole in the disk comes
before the light when the blade has completed half its movement in one
direction, while the second hole comes into line with the light when
the blade is at the same point on its return journey. Consequently the
light falls upon the blade at the same spot every time, causing the
eye to imagine that it sees the blade always in the one position as
if under a steady ray of continuous light. Hence comes its apparent
quiescence. But directly the speed of the motor is altered in relation
to the vibration of the tuning-fork, the rays of light catch the blade
at varying points in its travel, and these changes, coming in quick
succession, convey the visual idea of movement. Acceleration of the
motor so that its revolving speed per second exceeds the number of the
tuning-fork vibrations, causes the perceptible movements to be made
more quickly, while on the other hand deceleration slows them down. In
reality the eye imagines that it sees more than what actually takes
place; it imagines that the blade of the fork is kicking spasmodically
and viciously, whereas in fact the extent of the movement to and fro is
constant and never changes.
While the experiment is peculiarly fascinating, its application is
extremely useful to the worker. It offers a means of being absolutely
certain about the speed at which the instrument utilised in a
particular investigation is running, so that the resulting calculations
may be completed without the slightest error.