1961 Westside Dr., Rochester, New York 14624.
'Remember those movies we took of the tractors at the Reunion? Well, they are good except the wheels are going backwards like crazy!' Be not discouraged. The professionals have the same problem. In Western movies the wheels of the stage coaches usually appear to go backwards some of the time.
Whenever we view by stroboscopic (intermittent) vision, a rotating member that has a pattern which is repetitive with rotation such as a wheel with spokes or lugs, curious effects are usually observed. Movie cameras do not record continuously but snap 'stills' at discrete intervals. The time intervals between 'stills' are utilized to advance the film to the next frame. Let us examine in sequence the effects that may be observed as when a wheel with spokes for example, is viewed in movies as it is accelerated from a position of rest. As the wheel starts to turn, the spokes as viewed, advance in position in successive frames and the direction and rotation rate are as observed. As soon however as the rotation is fast enough to bring the positions of the spokes in a frame to more than half way to the positions of the NEXT spokes as they were shown in the PRECEDING frame, the motion of the wheel appears to change from rapidly ahead to rapidly backwards.
The observed (not actual) reversal of motion occurs because the eye in interpreting what it sees, automatically identified (relates) positions of spokes in one frame, not with the positions occupied by them in the preceding frame, but with the positions of the NEXT spokes because they are closer to the positions the next ones had in the preceding frame. The spokes therefore seem to move backwards. As the rotational speed continues to increase, the backwards displacement between successive frames lessens and the apparent backward speed is thereby reduced. When the wheel rotates rapidly enough so that one spoke occupies exactly the same position as the one preceding it did in the preceding frame then the wheel appears to stop.
The cycle of the sequence of apparent motion i.e. acceleration forward, reversal, and deceleration to a stop, may be repeated several times as the wheel gains speed. The repetition of the sequence is caused by spokes becoming identified not with themselves and the next ones as detailed herein, but with the 2nd & 3rd, the 3rd & 4th spokes ahead, etc. At no time is the apparent speed faster in either direction than half a pattern of one spoke to the next per frame. Successive 'stops' indicate the rotational speeds have become two, three or more, whole intervals of spoke displacement per time lapse between successive frames.