Effect of changes in fin-length on apparent shaft-length and depth in the Müller-Lyer illusion.

Müller-Lyer figures with outgoing fins were back illuminated and apparent shaft-length and depth were both judged by 20 undergraduates. With the angle between the fins constant, progressive increases in fin-length produced first an increase in the apparent shaft-length and then a decrease. Changes in fin-length, however, had no significant effect on apparent depth. These data were interpreted as inconsistent with an account of the Müller-Lyer illusion in terms of perspective theory, since the latter predicts systematic changes in perceived shaft-length to be associated with systematic changes in perceived depth.