BINOCULAR VISION. 535 



after removing the arm or letting it down as far as possible. Place 

 the stand on a table which stands freely in the middle of the room, 

 take into your hand the lower part of a walking stick which has a 

 crook handle, shut one eye, approach the retort-stand from a 

 distance of several metres, and attempt to put the crook through 

 the ring by stretching out the arm which carries the stick. At 

 almost every trial the crook will either fall short of the ring or be 

 stretched beyond it ; but if both eyes are used, the crook is easily 

 put into the ring at once. 



The ring should be fixed nearly in a horizontal line with the head 

 of the experimenter, or very little lower. If the ring be suspended 

 by a fine thread from the ceiling, it is still more difficult to put the 

 crook into it. If the ring is suspended in this manner, it must first 

 be allowed to settle in a steady position before the experiment is 

 made. 



An accurately drawn picture of an object makes the 

 same impression upon a single eye, at least as regards 

 the external form, as the object itself, for one eye is as 

 little able to estimate directly the relative distances 

 of the various parts of the object as the picture is 

 of giving a direct representation of these distances. As 

 a consequence of this a picture produces the greatest 

 resemblance to reality when viewed with one eye only ; 

 as soon as the other eye is opened, the picture pre- 

 sents of course the same aspect to either eye, and the 

 difference between it and the appearance of the real 

 objects is at once detected. Thus after viewing with 

 one eye the picture of a church interior, or of a row 

 of pillars, the depth of the space represented will 

 make the impression of reality. This illusion, how- 

 ever, will at once be destroyed on opening the other 

 eye ; the parts of the picture which form the foreground 

 appear to recede, the background seems to move for- 

 ward, and the whole merges into the flat surface of 

 the picture. 



