230 PSYCHOLOGY. 



Let O be the point looked at, M an object farther, and 

 N an object nearer, than it. Then M and N will send the 

 lines of visible direction MM and NN to the two retinse. 

 If N be judged as far as O, it must necessarily lie where 

 the two lines of visible direction NN intersect the plane of 

 the arrow, or in two places, at W and at W. If M be 

 judged as near as O, it must for the same reason form two 

 images at M^ and M". 



It is, as a matter of fact, true that we often misjudge 

 the distance in the way alleged. If the reader will hold his. 

 forefingers, one beyond the other, in the median line, and 

 fixate them alternately, he will see the one not looked at,, 

 double ; and he will also, notice that it appears nearer to the 

 plane of the one looked at, whichever the latter may be, 

 than it really is. Its changes of apparent size, as the con- 

 vergence of the eyes alter, also prove the change of ajDpa- 

 rent distance. The distance at which the axes converge 

 seems, in fact, to exert a sort of attraction ujjon objects 

 situated elsewhere. Being the distance of which we are 

 most acutely sensible, it invades, so to speak, the whole 

 field of our perception. If two half-dollars be laid on the 

 table an inch or two apart, and the eyes fixate steadily the 

 point of a pen held in the median line at varying dis- 

 tances between the coins and the face, there will come a 

 distance at which the pen stands between the left half- 

 dollar and the right eye, and the right half-dollar and the 

 left eye. The two half-dollars will then coalesce into one ;, 

 and this one w411 show its apparent approach to the pen- 

 point by seeming suddenly much reduced in size.* 



Yet, in spite of this tendency to inaccuracy, we are never 

 actually mistaken about the half-dollar being behind the 

 pen-point. It may not seem far enough off, but still it is 

 farther than the point. In general it may be said that 

 where the objects are known to us, no such illusion of dis- 

 tance occurs in any one as the theory would require. And 

 in some observers, Hering for example, it seems hardly to 

 occur at all. If I look into infinite distance and get my 

 finger in double images, they do not seem infinitely far off. 



* Naturally it takes a smaller object at a less distance to cover by its 

 image a constant amount of retinal surface. 



