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AN AMERICAN TEXT- BOOK OF PHYSIOLOGY 



45° with the horizon, since in this position the eye appreciates their real 

 position less accurately than when they are vertical or horizontal. It is 

 diminished, but does not disappear, when the eye, instead of being allowed 



Fig. 165.— To illustrate contrast in space-perception (Muller-Lyer). 



to wander over the figure, is fixed upon any one point of the field of vision. 

 Hence the motions of the eye must be regarded as a factor in, but not the 

 sole cause of, the illusion. 



Fig. 166.— To illustrate contrast in space-perception (Muller-Lyer). 



The illusion in Fig. 164, where the line d is the real and the line / the 

 apparent continuation of the line a, is to be explained partly by the over- 

 estimation of acute angles and partly, according to Helmholtz, by irradiation. 



Fig. 167.— To illustrate contrast in space-perceptiim (Muller-Lyer). 



The fact that the illusion is greatly diminished by turning the figure on its 

 side seems to show that the tendency to over-estimate vertical dimensions 

 also plays a part in its production. 



