Fig. 1.
Fig. 2.
Aërial perspective has a great influence on the determination of these phenomena, particularly such as occur in the open and at great distances. The influence is to be recognized through the various appearances of distant objects, the various colors of distant mountains, the size of the moon on the horizon, and the difficulties which aërial perspective offers painters. Many a picture owes its success or failure to the use of aërial perspective. If its influence is significant in the small space of a painting, the illusions in nature can easily become of enormous significance, particularly when extremes are brought together in the observations of objects in unknown regions. The condition of the air, sometimes foggy and not pellucid, at another time particularly clear, makes an enormous difference, and statements whether about distance, size, colors, etc., are completely unreliable. A witness who has several times observed an unknown region in murky weather and has made his important observation under very clear skies, is not to be trusted.
An explanation of many sensory illusions may be found in the so-called illusory lines. They have been much studied, but Zöllner[313] has been the first to show their character. Thus, really quite parallel lines are made to appear unparallel by the juxtaposition of inclined or crossing lines. In figures 1 and 2 both the horizontal lines are actually parallel, as may be determined in various ways.
The same lines looked at directly or backwards seem, in [Fig. 1], convex, in [Fig. 2] concave.
Fig. 3.