the horse and the bird in the cage.  It cannot be otherwise.  It is simply the result of the positive afterimages.  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 afterimage, 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 zooetrope, 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 zooetrope 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 Stricker, 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