Abstract: A multistage evaluation method was used to measure visual illusion.It is evident that there is,on the one hand,a good correlation betweenthe multistage evaluation method and classical psychophysical one forthe measurement of vertical-horizontal illusion and on the other handthose data obtained by both methods were randomly distributed on the line ofzero up and down for comparing the length of both horizontal lines exposed. Strength of illusion was defined as follows: x_i=cetagories(-2,-1,0,+1,+2) x_0=ratio of comparative line in length to standardized one in aConfiguration The investigation of the vertical-horizonral illusion discovered that thestrength of illusion varies with change in the ratio of comparative lines inlength to the Standardized one in the configuration.The illusion is strongestwhen the ratio is 0.8. Furthermore,the results obtained also showed that each configuration inquestion had a distribution of possibility in strength of illusion.Whereby it issuitable to describe such distribution in fuzzy sets.

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