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AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



of a line placed vertically to the plane of the paper does not entirely dis- 

 appear when one eve is closed. Hence it is evident that there is, as Mrs. 



Fig. 177.— Monocular illusion of vertical lines. 



C. L. Franklin has pointed out, 1 a strong tendency to regard 

 lines which form their images approximately on the vertical 

 meridian of the eye as themselves vertical. This tendency 

 is well shown when a number of short lines converging 

 toward a point outside of the paper on which they are 

 drawn, as in Figure 177, are looked at with one eye held 

 a short distance above the point of convergence. Even 

 when the lines are not convergent, but parallel, so that their 

 images cannot fall upon the vertical meridian of the eye, the 

 illusion is not entirely lost. It will be found, for instance, 

 that when the Zollner lines, as given in Figure 163, are 

 looked at obliquely with one eye from one corner of the 

 figure, the short lines which lie nearly in a plane with the 

 visual axis appear to stand vertically to the plane of the 

 paper. 

 In this connection it may be well to allude to the optical illusion in conse- 

 quence of which certain portraits seem to follow the beholder with the eyes. 

 This depends upon the fact that the face is painted looking straight out from 

 the canvas — i. e. with the pupil in the middle of the eye. The painting being 

 upon a flat surface, it is evident that, from whatever direction the picture is 

 viewed, the pupil will always seem to be in the middle of the eye, and the 

 eye will consequently appear to be directed upon the observer. The phenom- 

 enon is still more striking in the case of pictures of which the one repre- 

 sented in Figure 178 may be taken as an example. Here the soldier's rifle 

 1 Am. Journal of Psychology, vol. i. p. 99. 



Fig. 176.— Binocu- 

 lar illusion of a ver- 

 tical line. 



