I’m currently reading Hugo Münsterberg’s fascinating 1916 book, The Photoplay (I’m reading a paper copy, but the link takes you to the complete online text). It’s one of the earliest serious works on film, which was unfortunately not well received at the time it was published due to the start of World War I and Münsterberg’s strong German nationalism (he was a professor at Harvard at the time).
Anyway, I wanted to direct your attention to a couple descriptions of amazing research conducted with the extremely limited tools available at that time:
If a flash of light at one point is followed by a flash at another point after a very short time, about a twentieth of a second, the two lights appear to us simultaneous. The first light is still fully visible when the second flashes, and it cannot be noticed that the second comes later than the first. If now in the same short time interval the first light moves toward the second point, we should expect that we would see the whole process as a lighted line at rest, inasmuch as the beginning and the end point appear simultaneous, if the end is reached less than a twentieth of a second after the starting point. But the experiment shows the opposite result. Instead of the expected lighted line, we see in this case an actual movement from one point to the other. Again we must conclude that the movement is more than the mere seeing of successive positions, as in this case we see the movement, while the isolated positions do not appear as successive but as simultaneous.
And then there’s this:
The recent experiments by Wertheimer and Korte have gone into still subtler details. Both experimenters worked with a delicate instrument in which two light lines on a dark ground could be exposed in very quick succession and in which it was possible to vary the position of the lines, the distance of the lines, the intensity of their light, the time exposure of each, and the time between the appearance of the first and of the second. They studied all these factors, and moreover the influence of differently directed attention and suggestive attitude. If a vertical line is immediately followed by a horizontal, the two together may give the impression of one right angle. If the time between the vertical and the horizontal line is long, first one and then the other is seen. But at a certain length of the time interval, a new effect is reached. We see the vertical line falling over and lying flat like the horizontal line. If the eyes are fixed on the point in the midst of the angle, we might expect that this movement phenomenon would stop, but the opposite is the case. The apparent movement from the vertical to the horizontal has to pass our fixation point and it seems that we ought now to recognize clearly that there is nothing between those two positions, that the intermediate phases of the movement are lacking; and yet the experiment shows that under these circumstances we frequently get the strongest impression of motion. If we use two horizontal lines, the one above the other, we see, if the right time interval is chosen, that the upper one moves downward toward the lower. But we can introduce there a very interesting variation. If we make the lower line, which appears objectively after the upper one, more intense, the total impression is one which begins with the lower. We see first the lower line moving toward the upper one which also approaches the lower; and then follows the second phase in which both appear to fall down to the position of the lower one. It is not necessary to go further into details in order to demonstrate that the apparent movement is in no way the mere result of an afterimage and that the impression of motion is surely more than the mere perception of successive phases of movement. The movement is in these cases not really seen from without, but is superadded, by the action of the mind, to motionless pictures.
Both of these experiments demonstrate that it is the human mind which creates the illusion of movement in a motion picture, which is, of course, nothing more than a sequence of still photos. Much of the research on perception and the experience of watching film we see today is merely an extension of this work that is now nearly a hundred years old.