Cited by T. Quick, 'Disciplining Physiological Psychology: Cinematographs as Epistemic Devices, 1897-1922', Science in Context 30 (4), pp. 423-474.
Description:

'Sherrington was not the only British physiologist to experiment with illusions of temporal continuity at this time. Shortly after Sherrington's initial flicker study came out, his Cambridge compatriot Otto Fritz Frankeau Grünbaum published a two-part paper in the Journal of Physiology (Grünbaum 1897-1898). This study, which also addressed the rate at which alternating stimuli fused into continuous perception, strove for far greater precision than had Sherrington. Rather than direct both eyes to an external stimulus in the shape of a rotating disc, Grünbaum created a mechanism (fig. 3) that could project two separate beams of light onto the retina of a single eye. One of these beams was constant, thus constituting a point of comparison. The other however was interrupted by a disc with angular segments cut out of it. By rotating this disc at different speeds, Grünbaum sought to arrive at a more accurate estimation of the rates of exposure necessary for the experiences of different brightnesses. The parallels between the establishment of illusion-generating devices within physiology laboratories and that of cinematographic technology more generally are prominent here. One of the key elements of the cinematograph - and one of the most difficult pieces to co-ordinate with the movement of celluloid film-strips - was a disc or 'shutter' (similar to the ones Talbot and von Stampfer had developed) that cut off the projection of light at intervals proportional to the replacement of images on the projector. The minimal rate at which cinematographic images had to be replaced and exposed to view was determined by the rate at which images could be perceived as individual, rather than a continuous moving picture - i.e. by the phenomena of persistence of vision. [note: 'Münsterberg made the connection explicit. See Münsterberg 1916, 57-71.'] During the 1890s, the physiological study of flicker fusion and the construction of cinematographic devices went hand in hand.'

 

Relevant passage from Münsterberg:

 

'While the problem of depth in the film picture is easily ignored, the problem of movement forces itself on every spectator. It seems as if here the really essential trait of the film performance is to be found, and that the explanation of the motion in the pictures is the chief task which the psychologist must meet. We know that any single picture which the film of the photographer has fixed is immovable. We know, furthermore, that we do not see the passing by of the long strip of film. We know that it is rolled from one roll and rolled up on another, but that this movement from picture to picture is not visible. It goes on while the field is darkened. What objectively reaches our eye is one motionless picture after another, but the replacing of one by another through a forward movement of the film cannot reach our eye at all. Why do we, nevertheless, see a continuous movement? The problem did not arise with the kinetoscope only but had interested the preceding generations who amused themselves with the phenakistoscope and the stroboscopic disks.

But this so-called stroboscopic phenomenon, however interesting it was, seemed to offer hardly any difficulty. The friends of the zootrope surely knew another little play-thing, the thaumatrope. Dr. Paris had invented it in 1827. It shows two pictures, one on the front, one on the rear side of a card. As soon as the card is quickly revolved about a central axis, the two pictures fuse into one. If a horse is on one side and a rider on the other, if a cage is on one and a bird on the other, we see the rider on th horse and the bird in the cage. It cannot be otherwise. It is simply the result of the positive after-images. If at dark we twirl a glowing joss stick in a circle, we do not see one point moving from place to place, but we see a continuous circular line. It is nowhere broken because, if the movement is quick, the positive afterimage of the light in its first position is still effective in our eye when the glowing point has passed through the whole circle and has reached the first position again.

We speak of this effect as a positive after-image, because it is a real continuation of the first impression and stands in contrast to the so-called negative afterimage in which the aftereffect is opposite to the original stimulus. In the case of a negative afterimage the light impression leaves a dark spot, the dark impression gives a light afterimage. Black becomes white and white becomes black; in the world of colors red leaves a green and green a red afterimage, yellow a blue and blue a yellow afterimage. If we look at the crimson sinking sun and then at a white wall, we do not see red light spots but green dark spots. Compared with these negative pictures, the positive afterimages are short and they last through any noticeable time only with rather intense illumination. Yet they are evidently sufficient to bridge the interval between the two slits in the stroboscopic disk or in the zootrope, the interval in which the black paper passes the eye and in which accordingly no new stimulus reaches the nerves. The routine explanation of the appearance of movement was accordingly: that every picture of a particular position left in the eye an afterimage until the next picture with the slightly changed position of the jumping animal or of the marching men was in sight, and the afterimage of this again lasted until the third came. The afterimages were responsible for the fact that no interruptions were noticeable, while the movement itself resulted simply from the passing of one position into another. What else is the perception of movement but the seeing of a long series of different positions? If instead of looking through the zootrope we watch a real trotting horse on a real street, we see its whole body in ever new progressing positions and its legs in all phases of motion; and this continuous series is our perception of the movement itself.

This seems very simple. Yet it was slowly discovered that the explanation is far too simple and that it does not in the least do justice to the true experiences. With the advance of modern laboratory psychology the experimental investigations frequently turned to the analysis of our perception of movement. In the last thirty years many researches, notably those of Strieker, Exner, Hall, James, Fischer, Stern, Marbe, Lincke, Wertheimer, and Korte have thrown new light on the problem by carefully devised experiments. One result of them came quickly into the foreground of the newer view: the perception of movement is an independent experience which cannot be reduced to a simple seeing of a series of different positions. . A characteristic content of consciousness must be added to such a series of visual impressions. The mere idea of succeeding phases of movement is not at all the original movement idea. This is suggested first by the various illusions of movement. We may believe that we perceive a movement where no actual changes of visual impressions occur. This, to be sure, may result from a mere misinterpretation of the impression: for instance when in the railway train at the station we look out of the window and believe suddenly that our train is moving, while in reality the train on the neighboring track has started. It is the same when we see the moon floating quickly through the motionless clouds. We are inclined to consider as being at rest that which we fixate and to interpret the relative changes in the field of vision as movements of those parts which we do not fixate.

But it is different when we come, for instance, to those illusions in which movement is forced on our perception by contrast and aftereffect. We look from a bridge into the flowing water and if we turn our eyes toward the land the motionless shore seems to swim in the opposite direction. It is not sufficient in such cases to refer to contrasting eye movements. It can easily be shown by experiments that these movements and counter-movements in the field of vision can proceed in opposite directions at the same time and no eye, of course, is able to move upward and , downward, or right and left, in the same moment. A very characteristic experiment can be performed with a black spiral line on a white disk. If we revolve such a disk slowly around its center, the spiral line produces the impression of a continuous enlargement of concentric curves. The lines start at the center and expand until they disappear in the periphery. If we look for a minute or two into this play of the expanding curves and then turn our eyes to the face of a neighbor, we see at once how the features of the face begin to shrink. It looks as if the whole face were elastically drawn toward its center. If we revolve the disk in the opposite direction, the curves seem to move from the edge of the disk toward the center, becoming smaller and smaller, and if then we look toward a face, the person seems to swell up and every point in the face seems to move from the nose toward the chin or forehead or ears. Our eye which watches such an aftereffect cannot really move at the same time from the center of the face toward both ears and the hair and the chin. The impression of movement must therefore have other conditions than the actual performance of the movements, and above all it is clear from such tests that the seeing of the movements is a unique experience which can be entirely independent from the actual seeing of successive positions. The eye itself gets the impression of a face at rest, and yet we see the face in the one case shrinking, in the other case swelling; in the one case every point apparently moving toward the center, in the other case apparent- ly moving away from the center. The experience of movement is here evidently produced by the spectator's mind and not excited from without.

We may approach the same result also from experiments of very different kind. 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 alighted 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.

Another group of interesting phenomena of movement may be formed from those cases in which the moving object is more easily noticed than the impressions of the whole field through which the movement is carried out. We may overlook an area in our visual field, especially when it lies far to one side from our fixation point, but as soon as anything moves in that area our attention is drawn. We notice the movement more quickly than the whole background in which the movement is executed. The fluttering of kerchiefs at a far distance or the waving of flags for signaling is characteristic. All indicate that the movement is to us something different from merely seeing an object first at one and afterward at another place. We can easily find the analogy in other senses. If we touch our forehead or the back of our hand with two blunt compass points so that the two points are about a third of an inch distant from each other, we do not discriminate the two points as two, but we perceive the impression as that of one point. We cannot discriminate the one pressure point from the other. But if we move the point of a pencil to and fro from one point to the other we perceive distinctly the movement in spite of the fact that it is a movement between two end points which could not be discriminated. It is wholly characteristic that the experimenter in every field of sensations, visual or acoustical or tactual, often finds himself before the experience of having noticed a movement while he is unable to say in which direction the movement occurred. We are familiar with the illusions in which we believe that we see something which only our imagination supplies. If an unfamiliar printed word is exposed to our eye for the twentieth part of a second, we readily substitute a familiar word with similar letters. Everybody knows how difficult it is to read proofs. We overlook the misprints, that is, we replace the wrong letters which are actually in our field of vision by imaginary right letters which correspond to our expectations. Are we not also familiar with the experience of supplying by our fancy the associative image of a movement when only the starting point and the end point are given, if a skillful suggestion influences our mind. The prestidigitator stands on one side of the stage when lie apparently throws the costly watch against the mirror on the other side of the stage; the audience sees his suggestive hand movement and the disappearance of the watch and sees twenty feet away the shattering of the mirror. The suggestible spectator cannot help seeing the flight of the watch across the stage.

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.

The statement that our impression of movement does not result simply from the seeing of successive stages but includes a higher mental act into which the successive visual impressions enter merely as factors is in itself not really an explanation. We have not settled by it the nature of that higher central process. But it is enough for us to see that the impression of the continuity of the motion results from a complex mental process by which the various pictures are held together in the unity of a higher act. Nothing can characterize the situation more clearly than the fact which has been demonstrated by many experiments, namely, that this feeling of movement is in no way interfered with by the distinct consciousness that important phases of the movement are lacking. On the contrary, under certain circumstances we become still more fully aware of this apparent motion created by our inner activity when we are conscious of the interruptions between the various phases of movement.

We come to the consequences. What is then the difference between seeing motion in the photoplay and seeing it on the real stage? There on the stage where the actors move the eye really receives a continuous series. Each position goes over into the next without any interruption. The spectator receives everything from without and the whole movement which he sees is actually going on in the world of space without and accordingly in his eye. But if he faces the film world, the motion which he sees appears to be a true motion, and yet is created by his own mind. The afterimages of the successive pictures are not sufficient to produce a substitute for the continuous outer stimulation; the essential condition is rather the inner mental activity which unites the separate phases in the idea of connected action. Thus we have reached the exact counterpart of our results when we analyzed the perception of depth. We see actual depth in the pictures, and yet we are every instant aware that it is not real depth and that the persons are not really plastic. It is only a suggestion of depth, a depth created by our own activity, but not actually seen, because essential conditions for the true perception of depth are lacking. Now we find that the movement too is perceived but that the eye does not receive the impressions of true movement. It is only a suggestion of movement, and the idea of motion is to a high degree the product of our own reaction. Depth and movement alike come to us in the moving picture world, not as hard facts but as a mixture of fact and symbol. They are present and yet they are not in the things. We invest the impressions with them. The theater has both depth and motion, without any subjective help; the screen has them and yet lacks them. We see things distant and moving, but we furnish to them more than we receive; we create the depth and the continuity through our mental mechanism.' (57-71)