Fig. 70.--The Poggendorf illusion. Are the two obliques parts of the same straight line?
Any oblique line, any complication of any sort, is pretty sure to alter the apparent proportions and directions of the figure. A broad effect, a long effect, a skewed effect, may easily be produced by extra lines suitably introduced into a dress, into the front of a building, or into a design of any sort; so that the designer needs to have a practical knowledge of this type of illusion.
Extra lines have an influence also upon esthetic perception. The esthetic effect of a given form may be quite altered by the introduction of apparently insignificant extra lines.
Fig. 71.--The barber-pole illusion. The rectangle represents a round column, around which runs a spiral, starting at a. Which of the lines, 1, 2, 3, 4, and 5, comes closest to being a continuation of a?
Esthetic perception is very much subject to the law of combination, and to the resulting difficulty of isolation.
One of the most interesting illusions, not being visual, can [{459}] only be described and not demonstrated here.