THE PERCEPTION OF SPACE, 215 



object does not change its distance or its front. But sup- 

 pose, to take a more complicated case, that the object is a 

 stick, seen first in its whole length, and then rotated round 

 one of its ends ; let this fixed end be the one near the eye. 

 In this movement the stick's image will grow progressively 

 shorter ; its farther end will appear less and less sepa- 

 rated laterally from its fixed near end ; soon it will be 

 screened by the latter, and then reappear on the opposite 

 side, and finally on that side resume its original length. 

 Suppose this movement to become a familiar experience j 

 the mind will presumably react upon it after its usual fash- 

 ion (which is that of unifying all data which it is in any 

 way possible to unify), and consider it the movement of a 

 constant object rather than the transformation of a fluctuat- 

 ing one. Now, the sensation of depth which it receives dur- 

 ing the experience is awakened more by the far than by the 

 near end of the object. But how much depth ? What shall 

 measure its amount ? Why, at the moment the far end is 

 ready to be eclipsed, the diti'erence of its distance from the 

 near end's distance must be judged equal to the stick's 

 whole length ; but that length has already been judged 

 equal to a certain optical sensation of breadth. Thus ice 

 find that given amounts of the visual depth-feeling become signs 

 of fixed amounts of the visual breadth-feeling. The measure- 

 ment of distance is, as Berkeley truly said, a result of sugges- 

 tion and experience. Bid visucd experience alone is adequate 

 to produce it, and this he erroneously denied. 



Suppose a colonel in front of his regiment at dress- 

 parade, and suppose he walks at right angles towards the 

 midmost man of the line. As he advances, and surveys 

 the line in either direction, he looks more and more doivn 

 it and less and less at it, until, when abreast of the mid- 

 most man, he feels the end men to be most distant ; then 

 when the line casts hardly any lateral image on his retina 

 at alL what distance shall he judge to be that of the end 

 men? Wh}^, half the length of the regiment as it was 

 originally seen, of course ; but this length was a moment 

 ago a retinal object spread out laterally before his sight. 

 He has now merely equated a retinal depth-feeling with a 

 retinal breadth-feeling. If the regiment moved, and the 



