Effects of Instructional Set and Physical Stimuli on the Mueller-Lyer Illusion Page: 58

                                
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the Mueller-Lyer figure was used.. The figures were drawn
with the center oblique varying in position across the middle
extent from 48 mm. to 66 mm. from the left oblique by incre-
ments of two millimeters.   The dependent variable, the
magnitude of the illusion, was measured by the mean number
of "longer" responses the subject gave to the right hand.
section of the Mueller-Lyer figure. Subjects saw the stimu-
lus figures in a dark room with a spot of sufficiently bright
light reflected on only the stimulus figure.
The data obtained were treated in a two by three analysis
of variance. The resultant F scores determined the effect of
the instructional set on the magnitude of illusion, Also, t
tests were applied to determine the effect of the stimulus
variables on the magnitude of the illusion and to obtain
additional information.
The results supported only Hypothesis Two. It was
suggested that the "apex line absent" figure acted as a con-
taminant to the confirmation of the other two hypotheses.
Discussion centered around the effects that the "apex line
absent" figure may have had on the results of the study.
Influences acting contrary to the "central tendency effect"
mechanism were suggested as possible explanations for the
results of the study.
Conclusions
Man's perception is influenced by two factors: (1) the
information he receives about the nature of the object to be