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 he 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