Size illusion, distance illusion, and terrestrial passage: comment on reed.

Two assumptions of Reed's (1984) terrestrial passage theory are questioned. First, Reed assumes that the moon's failure to increase in visual subtense while elevating is accounted for strictly by perceptual distancing. This allows a formal account of the moon distance illusion, but at the expense of a compelling explanation of the moon size illusion. Second, in order to explain the distance illusion, Reed assumes that all objects, regardless of their perceived altitude, are perceived to start from a common point at the horizon. Several alternative application of Reed's terrestrial-passage foundation to the actual illusions are suggested.