Dietz tells of color-illusions following upon insignificant indigestion; Foder
altered and changes in size, in form, or appearance occur.'' Naturally the criminalist can not perceive slight indigestion, weak hysteria, or an inflamed area in the retina when he is examining witnesses, yet false observations like those described may have a definite influence upon the decision in a case.
If such abnormal occasions are lacking the reasons for optical illusions are of another nature. As a rule optical illusions occur when there is an interruption in the communication between the retina, the sense of movement, and the sense of touch, or when we are prevented from reducing the changes of the retinal image to the movement of our body or of our eyes. This reduction goes on so unconsciously that we see the idea of the object and its condition as a unit. Again, it is indubitable that the movement of the body seems quicker when we observe it with a fixed glance than when we follow it with our eyes. The difference may be so significant that it is often worth while, when much depends on determining the speed of some act in a criminal case, to ask how the thing was looked at.
Fechner has made a far-reaching examination of the old familiar fact that things on the ground appear to run when we ride by them rapidly. This fact may be compared with the other, that when you look directly into swift-moving water from a low bridge, the latter seems all of a sudden to be swimming rapidly up stream, though the water does not appear to stand still. Here some unknown factor is at work and may exercise considerable influence on many other phenomena without our being able to observe the results. To this class may be added the extraordinary phenomenon that from the train objects easily seem too near and hence appear smaller than they are. It may be, however, that the converse is true and objects appear smaller, or at least shorter, and that inasmuch as we are in the habit of attributing the diminution of size in objects to their distance, we tax the latter as false. So much is certain--that whenever we ourselves move quickly we make false judgments of size, distance, and even color. The last may be due to the fact that during a quick passage, colors may so compose themselves, that green and red become white, and blue and yellow, green, etc. I believe that all these illusions are increasing in connection with the spread of bicycling, inasmuch as many observations are made from the fleeting wheel and its motion tends to increase the illusions considerably. Concerning the differences in movement Stricker
 Elemente die Psychophysik. Leipzig 1889.