Figure

The two stages of the playing card experiment, as discussed by Woodsworth and Schlosberg (1954). When the first setup is viewed, the card at the back looks further away, which it is. However, when the front card has been clipped and the position of the card rearranged, the back card looks as if it overlaps the front card. The cue of familiar size, telling the viewer that the smaller card must be further away from the bigger card, is overridden by the cue of interposition, suggesting that the card that appears to obscure part of the other one must be nearer to the viewer, despite its size.

Evidence consistent with the size-distance invariance hypothesis was reported by Holway and Boring (1941). Participants sat at the intersection of two hallways. The test circle was presented in one hallway, and the comparison circle was presented in the other one. The test circle could be of various sizes and at various distances, and the participants' task was to adjust the comparison circle so that it was the same size as the test circle. Their performance was very good when depth cues were available. However, it became poor when depth cues were removed by placing curtains in the hallway and requiring the participants to look through a peephole. Lichten and Lurie (1950) went a step further and removed all depth cues by using screens that only allowed the observers to see the test circles. In those circumstances, the participants relied totally on retinal image size in their judgements of object size.

If size judgements depend on perceived distance, then size constancy should not be found when the perceived distance of an object is very different from its actual distance. The Ames room provides a good example (see Figure 2.10). It has a peculiar shape: the floor slopes, and the rear wall is not at right angles to the adjoining walls. In spite of this, the Ames room creates the same retinal image as a normal rectangular room when viewed through a peephole. The fact that one end of the rear wall is much further from the viewer is disguised by making it much higher. The cues suggesting that the rear wall is at right angles to the viewer are so strong that observers mistakenly assume that two adults standing in the corners by the rear wall are at the same distance from them. This leads them to estimate the size of the nearer adult as being much greater than that of the adult who is further away.

Evaluation

Perceived size and size constancy do typically depend in part on perceived distance. However, the relationship between perceived distance and perceived size is influenced by the kind of size judgements that

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