Water, fog and the size-distance invariance hypothesis.

Abstract

Objects generally appear too large under water. The usual explanation for this is that they are magnified by the change of refractive index between air and water. However, they are only magnified because they are optically nearer. If size constancy operates normally they should be seen the correct size but nearer, or enlarged in size but farther than the optical distance. The problem was investigated by requiring sixteen divers to make size and distance judgements of a range of white disks of different sizes set at various distances. Judgements were made for the same display both under water and on land. It was found that size judgements were greater under water, and that this corresponded to relative overestimation of distance. A weak form of size—distance invariance was found both under water and on land, but the relation between size and distance judgements was not the same in both conditions.Overestimation of size and distance was also found in a fog on land. It was suggested that the overestimation of distance in a fog and under water was due to the exaggerated reduction of brightness contrast with distance, which increases ‘aerial perspective’.