248 ADAPTATIONS TO SPACE AND MOTION 



accurately evaluated. Thus, strangers in mountainous country underesti- 

 mate distance because of the exceptional clearness of the air — in their 

 experience, objects look that sharp only when they are nearer. Similarly, 

 distances through mist and fog are easily overestimated, since ordinarily 

 such ha^y outlines and unsaturated colors connote greater distances. 



An amusing and convincing demonstration of this interdependence of 

 size- and distance-judgements is the following experiment, which the 

 reader can make without travelling to the Rockies or waiting for a fog : 

 Stand before a large mirror with arm outstretched before you, index fin- 

 ger pointing upward. Watching the finger closely, move it toward you 

 and away from you, noticing your reflection in the mirror all the while 

 but without taking your attention off of your finger. As the finger ap- 

 proaches your face, your image in the mirror will appear to shrink and 

 recede; and when the finger is moved toward the mirror your reflection 

 will seem to advance and expand.* 



The point is that judgement of distance is very largely subjective and 

 is very easily deceived. This, despite the fact that the eye may make a 

 thoroughly objective and very precise adjustment to the distance of the 

 object to which we are giving attention, the adjustment called accom- 

 modation. If nature has anywhere fallen down very badly in designing 

 our visual mechanism, it is in neglecting to tie our awareness of distance 

 firmly to our neuromuscular apparatus for adjusting to it, which would 

 have put the estimation of distance upon an objective basis. However, 

 nature may be pardoned on this score so far as we humans are concerned, 

 for we have binocular vision at a maximum and gain a potent cue to dis- 

 tance, from the convergence which our eyes automatically perform along 

 with their accommodation, and which takes place even beyond the limit 

 of distance within which accommodation is necessary for most of us. 



It is not distance as such, or its variation as such, that makes accom- 

 modation necessary. Some animals get along nicely without it, even 

 though they may be standing alongside of others which are utterly de- 

 pendent upon accommodation for maintaining an equally sharp image 

 of an object at which both are looking. The need for accommodation, or 



*Explanation: As the finger nears your face its retinal image enlarges; but since your atten- 

 tion is steadily upon the finger, which is very familiar to you and whose size you know to be 

 constant, you see it as having the same size as ever. By contrast, however, the motionless 

 image of your whole body, over which the finger is superimposed, becomes relatively smaller 

 and is hence perceived as shrinking in size. And, since in your experience decreasing (or 

 apparently decreasing) size always means increasing distance, your image seems to recede 

 farther behind the mirror. As the finger is moved away these processes are reversed and the 

 mirror image apparently enlarges and comes toward you. 



