MOTION PICTURES.
Strictly speaking there are no pictures of motion. What we see as such
is simply an optical illusion. This illusion is produced by presenting a
series of pictures of an object in a systematic manner, each picture
showing some slight change from the preceding one. If these changes be
all in a certain direction and brought before our eyes in regular order,
we shall perceive the appearance of motion in that direction. Such
pictures may be made by means of photography.
A very simple form of motion picture is made up in the form of a small
book containing a number of leaves that may be run off under the finger
of the holder. If these leaves contain such a series of pictures as is
mentioned above, the holder, on manipulating them properly, will see
motion reproduced quite naturally.
The manner in which the illusion of motion is produced can perhaps best
be illustrated by Figure 27. Here we have an ordinary film, or it may be
any piece of white paper, upon which are drawn a series of black dots as
shown. If this film--the observer being able to see only that part in
the aperture _A_--be drawn downward the length of one section very
quickly; allowed to rest a moment; then, in the same manner, be drawn
down another section; and this process repeated at proper speed, until
the full length of the film has passed the aperture, we shall have
received the impression that the black dot moved from the lower
left-hand corner to the upper right-hand corner of the aperture. In
order that such an illusion might be perfect, we should have to move the
film so rapidly that the eye would not perceive the movement. This is
not possible except with very weak illumination and we should actually,
in the above experiment, receive a blurred impression, because we could
not help seeing the dots while they were moving, and our eyes would
behold a mixture of stationary and moving dots. In order to produce the
impression of perfect motion, it is necessary to shut off the light
during the time that the film is actually in motion. Thus, paradoxical
as it may seem, in order to simulate motion, we must have the object
which is to appear in motion always perfectly still before our eyes.
[Illustration: FIGURE 27.]
In order that we may not notice that the film is out of sight, it must
be moved very quickly. The actual time during which the picture on the
average film is hidden from view, and in which the picture is changed,
is about 1/80th of a second and the time during which the picture is
stationary is about 4/80th of a second.
The possibility of the illusion of motion pictures depends upon a
faculty of the eye known as _persistence of vision_. The eye retains an
impression for something like 1/25th of a second. When an object is in
motion, we see, therefore, not only one position of the object but all
positions of it during the time of persistence of vision. This time
varies somewhat with the intensity of the light or the impression made
upon the eye. If it is equal to 1/25th of a second in the case of a ball
thrown at the rate of one hundred feet per second, then we should see,
instead of one ball, a large number of balls merging imperceptibly into
one another, or, in other words, a streak of balls four feet long. Thus,
in actual life, we obtain from the moving ball but a blurred impression.
We see thus that in order to produce the impression of motion, we must
present the picture to the eye long enough to stimulate it properly; we
must very quickly remove that picture and substitute another differing
to a slight extent from the former; and we must repeat this process a
number of times. The ordinary moving picture film contains 16 pictures
per foot, and is run off at the rate of about 60 feet per minute, so
that in one minute, we see 960 different pictures.
In order to make motion visible, we must bring it within a certain speed
limit. Thus, to show the motion of a swiftly thrown ball in detail, we
must make it appear to move more slowly than it really does; and to show
the development of a growing plant, it must appear to grow much faster
than it actually does. Both of these requirements can easily be
fulfilled by the motion picture camera and the projecting machine.
A man, walking at the rate of three miles an hour, displaces himself
about three inches during the time of the exposure of one picture, or
1/16th of a second. At this rate we obtain the impression of even and
continuous motion unless he be too close to the camera. In order to
obtain pictures of other objects moving at faster or slower rates, we
must take them at intervals in order that the displacement between
pictures will be about the same or at least not any more. This means
that pictures of rapidly moving objects must be taken at short intervals
and those of slowly moving objects, at long intervals. A kernel of corn
develops into a stalk six feet high in about ninety days. If a
photograph of this is taken every day during its growth and these
pictures arranged in proper order, they will be run off at normal speed
in less than six seconds, thus showing us in six seconds the growth
which actually takes place in ninety days.
The motion picture camera enables us not only to produce the illusion of
motion, but to see in detail what actually takes place in connection
with the moving object at any instant. If we take pictures of a running
horse, for instance, at short enough intervals, we shall be able to see,
on the films, just how he holds or places his feet or any other part of
his body at any time.
In order to obtain a perfect picture simulating motion, we must present
the first picture long enough to stimulate the eye; then we must shut
off the light, remove the first picture, and substitute the second;
remove the second and substitute the third, etc., as long as desired.
During the time that the light is shut off, the first picture must
persist in our vision until the new one has appeared. The two pictures
thus mix until the first one has faded, and thus we obtain the illusion
of motion.
If the bright picture remains too long, the pupil contracts--as
explained in the chapter on Optics--and when next the light is shut off,
the darkness is noticeable and gives rise to the disagreeable phenomenon
of flicker. In order to prevent this over-stimulation of the eye, the
long period of exposure is interrupted by a shutter at least once and,
in some cases, two times; and some machines are equipped with a
three-blade shutter. This three-blade shutter has a wide blade which
shuts off the light while the film is in motion and two narrower blades
which pass across the light during the time that the film is stationary,
to prevent the over-stimulation of the eye.
_Colored Pictures._--There are two general methods of producing colored
motion pictures: One is that of hand coloring or tinting, and the other
is what is known as the _Kinemacolor process_. In the latter process, no
color whatever is used on the film; the coloring is supplied by a
shutter with a green and a red blade which are alternately thrust into
the light by which the picture is projected upon the screen.
In order that this process may be used, the film pictures must first be
taken through screens of corresponding color. The film in the
Kinemacolor camera, or projecting machine, must run at more than double
the speed of that which is used in the ordinary process; and each
alternate picture must be photographed through a red screen; the others,
through a green screen.
The red screen will allow only red light to pass; hence, any part of an
object that contains no red will not affect the photographic emulsion.
Similarly, the green screen will allow only green light to pass; and
such parts of the objects as contain no green will not affect the
emulsion. The alternate sections of film will thus be entirely
different from each other.
In order to reproduce the original color of the object upon the screen,
it is but necessary to arrange that the pictures shall in turn be
projected through the same or similar color screens. In order to
accomplish this, the Kinemacolor machine has, in addition to the regular
shutter which cuts off the light during the time the film is in motion,
an additional two-wing shutter which inserts the properly colored
screens before each picture, as it comes to a standstill in the film
window. Thus we see in alternation, a red picture and a green.
Persistence of vision, which is explained in Chapter IX, helps us to mix
the two colors and we see the object approximately in its own colors.
The color effect is not very good, owing partly to the fact that only
two colors are used. If the three primary colors--red, blue, and
yellow--could be used, the effect would doubtless be better; but the
complications would be multiplied.
In order to get the best results with the Kinemacolor process, colors of
a certain shade are used and the size and depth of coloring in the two
shutters is variable. One of the colors is adjustable and must be so
arranged that when the machine is run without film, it will throw
approximately white light upon the screen. The high speed with which the
film must be run makes it impossible to turn the machine by hand and it
is always motor-driven.
The colored shutters are constantly in the light and absorb a large
portion of it and this must be compensated for by an extraordinary high
amperage. This process requires more than two times as much current as
the ordinary projection of black and white pictures. In order to obtain
the greatest possible amount of light, operators usually run a very long
arc and this often results in imperfect definition.
In arranging for an exhibition, it is important that the film and the
colored screens be correctly placed with reference to each other; to
facilitate this, an identifying mark is placed on the side of one of the
colors.