Fig. 49.—An illusion of direction.
Fig. 50.—“Twisted-cord” illusion. These are straight cords.
Fig. 51.—“Twisted-cord” illusion. These are concentric circles.
The illusions in [Figs. 50] and [51] are commonly termed “twisted cord” effects. A cord may be made by twisting two strands which are white and black (or any dark color) respectively. This may be superposed upon various backgrounds with striking results. In [Fig. 50] the straight “cords” appear bent in the middle, owing to a reversal of the “twist.” Such a figure may be easily made by using cord and a checkered cloth. In [Fig. 51] it is difficult to convince the intellect that the “cords” are not arranged in the form of concentric circles, but this becomes evident when one of them is traced out. The influence of the illusion is so powerful that it is even difficult to follow one of the circles with the point of a pencil around its entire circumference. The cord appears to form a spiral or a helix seen in perspective.