928 



A MANUAL OF PHYSIOLOGY 



zontal lines are really parallel, but do not appear so owing to the 

 confusion of judgment produced by the short sloping lines. In 

 Fig. 406 the spaces covered by A, B, and C are equal squares, 

 but A appears taller than B, and C smaller than either A or B. 

 In the same figure the lines D and E are of the same length, but 

 E seems considerably longer than D. 



Illusions of movement are among the most interesting optical 

 illusions. If two similar objects are momentarily shown to the 

 _ ___^___ ___ ___ __ e y e i R ra pid succes- 



sion and at points 

 in space not separ- 

 ated by too great a 

 distance, the illu- 

 sion is produced 

 that the first object 

 has moved to the 

 position of the 

 second. Such illu- 



U\U\U\UU\UU) 



flUIffffffffffffft 



WfWffMMffff 



sions are the basis 



FIG. 405. ILLUSION OF PARALLEL LINES (ZOLLNER). of the SO - called 



' moving pictures ' 



shown by the cinematograph. A series of instantaneous photo- 

 graphs of a movement are taken, recording the successive 

 positions assumed by the moving body. When these are thrown 

 on the retina in the same order and in rapid succession, an 

 illusion of the original movement is produced. 



The apparent size and form of an object is intimately related 

 to the size, form, and sharpness of its image on the retina. We 

 are, therefore, 



able to discrimi- ABC 

 nate with great 

 precision the un- 

 stimulated from 

 the excited por- 

 tions of that 

 membrane, espe- 

 cially in the f ovea 

 centralis, and also 

 the degree of ex- 



citation of neighbouring excited parts. But instead of localizing 

 the image on the retina as we localize on the skin the pressure 

 of an object in contact with it, we project the retinal image 

 into space, and see everything outside the eye. 



In vision, in fact, we have no conception of the existence of either 

 retina or retinal image ; and even the shadows of objects within the 

 eye are referred to points outside it. Thus, for instance, an opacity 



FIG. 406. ILLUSIONS OF SPACE-PERCEPTION. 



