382 THE SPECIAL SENSES. 



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, 

 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. 163 the lines A and B 

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

 in Fig. 164 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 



Fig. 163. Muller-Lyer figures to show illusion in space perception. The lines A and B 



are of the same length. 



that are obtuse. A very remarkable delusion is given by Fig. 165. 

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

 inches from the face and the eyes are focused on the point of 

 intersection of any two of the lines, a third line will be seen per- 

 pendicular to the plane of the other two, and projecting vertically 

 from the surface of the page. A row of these vertical lines will 

 be seen if the distance is properly chosen. As one bends the 

 head from side to side the lines sway in the same direction. It 

 forms a very striking instance of the fact that we may see most 

 distinctly a thing that has no real existence, a case, therefore, 

 in which we can not trust our senses. The delusion seems to be 

 due to the fact that the two lines, in the position indicated, form 

 a projection on the retina such as would be made by an actual 

 vertical rod placed at the point at which we see one. Fig. 166 

 gives an interesting illustration of the way in which our judg- 



