350 THE SPECIAL SENSES. 



shown in the case of after-images (see p. 323). When an after- 

 image of any object is obtained on the retina our judgment of its 

 size depends altogether on the distance to which we project it. 

 If we look at a surface near at hand, it seems small ; if we gaze at a 

 wall many feet away it is at once greatly enlarged. The familiar 

 instance of. the variation in the size of the full moon according as it 

 is seen at the horizon or at the zenith depends upon the same fact. 

 The distance to the horizon as viewed along the surface of the earth 

 seems greater than to the zenith ; we picture the heavens above us 

 as an arched dome flattened at the top, and hence the same size of 

 retinal image is interpreted as larger when we suppose that we see 

 it at a greater distance. Our judgments of distance, on the other 

 hand, depend primarily upon the data already enumerated in 

 speaking of the perception of solidity or depth in the visual field. 

 For objects within the limit of accommodation we depend chiefly 

 on the muscle sense aroused by the act of focusing the eyes, that 

 is, the contractions of the ciliary and of the extrinsic muscles. For 

 objects outside the limit of accommodation we are influenced by 

 binocular perspective, mathematical perspective, aerial perspective, 



Fig. 153. Muller-Lyer figures to show illusion in space perception. The lines A and B 



are of the same length. 



etc. But here again our judgment of distance is greatly influenced 

 in the case of familiar objects by the size of the retinal image. A 

 striking instance of the latter fact is obtained by the use of field 

 glasses or opera glasses. When we look through them properly 

 the size of the retinal image is enlarged, and the objects, therefore, 

 seem to be nearer to us. If we reverse the glasses and look through 

 the large end the size of the retinal image is reduced and the objects, 

 therefore, seem to be much farther away, since under normal condi- 

 tions such small images of familiar objects are formed only when 

 they are at a great distance from us. 



Optical Deceptions. Wrong judgments as regards distance 

 and size are frequently made and the fact may be illustrated in a 

 number of interesting ways. Thus, in Fig. 153 the lines A and B 

 are of the same size, but B seems to be distinctly the longer. So 

 in Fig. 154 the vertical lines, although exactly parallel, seem, on 

 the contrary, to run obliquely with reference to one another. Both 

 of these deceptions depend apparently upon our inability to estimate 

 angles exactly ; we undervalue the acute angles and overvalue those 

 that are obtuse. A very remarkable delusion is given by Fig. 155. 

 If the book is held flat at the level of the chin and six or eight 



