Observations and results
In the first flip-book (which skipped no line spaces), did it look like the circles moved to the bottom of the card, then back to the top, and so on? In the second flip-book, did the circles appear to move differently, such as by jumping around or flashing?
When no lines are skipped, the circles in the flip-book should have clearly appeared to move down, then up, then back down again. Our brains can easily fill in some blanks and make sense of this motion when such a relatively short distance is skipped between images (for example, cards). In the second flip-book, however, three lines were skipped between images and this was probably too great a distance for the brain to decide the motion should be a smooth up-and-down one. Instead, the circles may have appeared to jump around or even flash, especially if not every card was viewed when flipping. In other words, the brain perceived apparent motion with the first flip-book, but most likely did not perceive this phenomenon with the second flip-book.
More to explore
Experimenting with Simple Animation Devices , from Brian Wesley Rich's Science Web Site
Development of Stop-Motion Animation , from Stop Motion Animation
Perception in Animation ( pdf ), from the University of Southern California Cinematic Arts
Apparent Motion and Animation , from Science Buddies
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