A horizontal line (L) looks shorter with large boxes (B) at the ends than with small Bs. This illusion was mea~ured by having Ss judge the length of L on a 6-point scale. Six different lengths of L were combined factorially with six sizes of B to fimll ]6 stimuli. and 51 Ss made four judgments each. Mean judgment increased slIloothly with L and decreased with B. Data were fitted using Adaptatioll-Level formulas. and the weight of B was found to be approximately 14%. However. B had almost no effect on the judgment of very short Ls.The full theoretical significance of geometrical illusions is obscured by the lack of accurate measurements of their magnitudes. The main methods of measurement call upon S to compare two displays, each of which may be relatively complex, and which may interact. The point of subjective equality is usually welI defmed experimentally, but it is the product of many complex factors.This paper reports measurements of a familiar relative-size illusion by the method of single stimuli. On each trial, a display like that shown in Fig. I is presented, and S judges the length of the line (L) between boxes (B). The effect of B can therefore be measured relative to the effect of L, and one can estimate the change in L required to compensate for the effects of a given change in B. Thus, a measurement of the illusion is obtained in physical amounts.Adaptation-Level (AL) theory (Helson, 1964) states that the size of an object is judged relative to the sizes of other objects in the immediate surround. The fundamental equation is J A (X) = XI A, where A is the adaptation level, X is the physical

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Contour-repulsion concepts of satiation, fatigue, or inhibition explain some illusions, but not the fact, shown in Exp. I, that a hashmark between two squares of unequal size is perceived closer to the larger square. The aforementioned illusion could be explained as a result of distance perspective, for the larger square might be perceived as closer. This perspective theory cannot explain the fact that a vertical hashmark, placed between two vertical lines of unequal length, appears closer to the long line. Both results agree in quantitative detail with an adaptation-level model saying that the length of a test extent is judged relative to other nearby extents.

4 groups of 32 female Ss each were compared on a 2-category concept-identification task with 6 stimulus dimensions. At the beginning of the task, 1 group received 6 trials on which E said "right" regardless of S's responses, 1 group received 6 "wrongs," and 1 group received 6 trials on which E said nothing. Another group received no prior trials. The group given 6 "rights" was significantly inferior to the other 3 groups in terms of trial of last error and learning rate parameters of the all-or-none model. The other 3 groups did not differ significantly from one another. It was suggested that noncontingent "rights" contribute maximally to random reinforcement effects.

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