A picture-plane theory of the illusion of parallelism was tested by replicating, with only standard and variable dimly visible, portions of a previous study demonstrating positive and negative directions of the illusion. The differences between means of the two studies ranged from 0.2° to 11.6°. Correlation between the matrices of the two studies was +.62, indicative of common determinants, one of which was proposed to be the aptitude of spatial visualization. The picture-plane theory remained tenable.
