SENSE OF SIGHT. 



595 



sun, but not long enough to produce fatigue, and then closed, 

 a bright spot of light is seen : this is a positive after-image. It 

 remains bright for but a short time and then changes color, 

 becoming greenish blue or bluish 

 green, blue, violet, purple, and red, 

 and then fading away entirely. It 

 may be followed by a negative after- 

 image. 



Visual Judgment. -We have al- 

 ready referred to some visual judg- 

 ments as to form, size, distance, 

 etc. (p. 584). It is a common say- 

 ing that " seeing is believing," and 

 yet not one of the senses is more 

 liable to deceive its possessor than 

 that of sight. For instance, if the 

 vertical and horizontal lines in Fig. 

 367 are compared, the vertical will 

 immediately be pronounced the 

 longer, and yet when accurately 

 measured it will be found that each is exactly 4 cm. in length. 

 This tendency to overestimate vertical lines is attributed to the 

 relative weakness of the superior rectus muscle as compared with 

 the muscles that move the eyeball horizontally. The difference is 

 said to be from 30 to 50 per cent, in height and 40 to 53 per 



FIG. 367. To illustrate the overesti- 

 mation of vertical lines. 



FIG. 368. To illustrate the illusion of subdivided space. 



cent, in area of cross-section ; owing to this weakness a greater 

 effort is required to turn the eyeball upward, and the effect upon 

 the mind is that of turning it through a greater distance ; hence 

 vertical lines seem to be longer than they really are. 



