Motion Picture Story Magazine (Feb-Jul 1912)

ECCENTRIC WAGON-WHEELS * v. inqu.ry ed.™ Some months ago, the question of why a wagon-wheel appears to be revolving backward when it should be turning forward was answered rather fully in "Answers to Inquiries," but since then the circulation of The Motion Picture Story Magazine has more than doubled, and many of our newer readers have asked the same question. This is the answer : The wagon-wheel that appears to be running forward, backward, or which runs first one way and then the other when the wagon steadily moves forward is one of the curiosities of cinematography, and has puzzled many. It must be remembered that the cinematograph camera does not picture continuous motion, but merely gives a series of consecutive pictures of motion, each one in itself motionless, but presented at such a speed that the illusion of movement is created thru persistence of vision. The eye and mind retain an impression of sight for a period of about one-tenth of a second, and, if in a second, more than ten pictures showing some continued motion are presented to the eye, the retina retains them all, but blends them into a whole, so: that the illusion of movement is created thru pictures without motion. It has been found that the best results are attained when sixteen pictures are presented to the eye each second, and the camera mechanisms are operated to record sixteen pictures a second on the strip of negative film. If a man walks across the picture stage, taking four steps a second, the camera records four pictures of each step, catching the leg on the ground, at two points in the air and on the ground again. It is perfectly possible with the modern camera to make six or eight or ten or twenty pictures of the leg in motion during each step, but it is not necessary, and to use the printer for our artist, we will suppose that these four periods mark the four points at which the action was caught : . . ., each dot representing one of the four pictures in its relation to the others. This is all that the eye and mind see, but the mind retains a picture of the first dot for an instant after the second dot and the second dot laps the third, and so on. Imagination aids in the illusion and carries the action from dot to dot, with the result that it looks like this instead of like this .... figure. You think you see the leg moving ahead because you know it is moving ahead, and you see nothing to make you think to the contrary. The 'action is continuously forward, and the illusion of actual movement is perfect. But suppose that, instead of a man's legs, we are watching the wheel of a wagon that has sixteen spokes. Suppose that these spokes are numbered from one to sixteen, starting with spoke number "one" at the top of the wheel, and numbering toward the rear, and so around until we come to spoke sixteen, immediately in front of spoke number one. Let us further suppose that this wheel is moving forward at the rate of one exact revolution each second, and that the camera is being turned at precisely the proper speed to make sixteen pictures each second. It follows that the first picture will show spoke number one, at the top of the wheel, and the second picture, spoke number two, in precisely the same position occupied by number one in the first picture. In the third picture, spoke number three will be at the top, and so on, until the seventeenth picture shows spoke number one in its original place. The wagon has moved forward a distance equal to the circumference of the wheel, and the wheel itself has made one full revolution, but, since each time 'a picture was made the spokes occupied the same relative position, the wheel appears to have been dragged, instead of rolled, forward. Each time you've seen a picture, you have seen another spoke at the top, but since they are all alike, and the sixteen spokes always occupy one, of sixteen positions, you do not get the illusion of motion. Now, suppose that, while the camera continues to take sixteen pictures a second, the wheel is revolved more slowly, taking seventeen-sixteenths seconds to make the full revolution. Now, picture number one will show spoke number one at the top of the wheel, but the second picture will show spoke two about one-sixteenth of the distance between spoke centers to the rear of the top, since the wheel does not revolve so rapidly, and the second picture is made before spoke two has had time to get to the top of the wheel. The third exposure will show spoke three a little further back of the position occupied by spoke two in the second picture, and each successive picture will show the next spoke still further back, like this : 1, 2, 3, 4. Now you have the perfect illusion -of a wheel revolving, but it is going backward, while the vehicle advances because the imagination draws the line from 1 to 4 in the direction in which this type runs. Reversing the idea, and supposing that the wheel makes a turn in fifteen-sixteenths of a second, each succeeding picture will show a spoke slightly ahead of the position occupied in the last picture, and the imagination supplies the illusion of forward motion, since it draws the line from one to four in this diagram : 4, 3, 2, 1, instead of this : 1, 2, 3, 4. But the apparent forward or backward movement does not depend upon the speed with which the wheel is revolved. The simplest illustration has been used above for the sake of clearness. The same illusion of backward motion will be created if the wheel is speeded up to a revolution of seventeen thirty-seconds of a second. In that case, instead of seeing spoke two in the position occupied by spoke one, we would see spoke three, since the movement is twice as rapid. The figure would be this : 1, 3, 5, 7, etc. It is not a matter of slow or rapid speed, but of the positions into whiph the spokes are moved. If the revolution, whether it be slow or fast, leaves the spokes where they are apparently, just ahead of where they were a moment before, the wheels appear to revolve forward. A slight increase or decrease in the speed will cause the wheels to seem to move backward, and a still further increase or decrease will again give the semblance of advancing movement, which explains how it is that, when a wagon first begins to move, the wheels appear to run first one way and then the other as the vehicle gains in speed. Tie a rag around one of the spokes to serve as a guide for the eye, and the imagination and the illussion is preserved, no matter how the spokes are placed, because now you watch that one spoke in its circuit of the wheel, and imagination does the rest. 132