DISCRIMINATION AND COMPARISON. 513 



I offer the explanation only as a partial one : it certainly 

 is not complete. Take the way in which practice refines 

 our local discrimination on the shin, for example. Two 

 compass-points touching the palm of the hand must be 

 kept, say, half an inch asunder in order not to be mistaken 

 for one point. But at the end of an hour or so of practice 

 with them we can distinguish them as two, even when less 

 than a quarter of an inch apart. If the same two regions 

 of the skin were constantly touched, in this experience, 

 the explanation we have been considering would perfectly 

 apply. Sujjpose a line ah c d ef of points upon the skin. 

 Suppose the local difference of feeling between a and / to 

 be so strong as to be instantly recognized when the points 

 are simultaneously touched, but suj^pose that between c and 

 d to be at first too small for this purpose. If we began by 

 putting the compasses on a and / and gradually contracted 

 their opening, the strong doubleness recognized at first 

 would still be suggested, as the comi^ass-points approached 

 the positions c and d ; for the point e would be so near/, and 

 so like it, as not to be aroused without/also coming to mind. 

 Similarly d would recall e and, more remotely,/. In such 

 wise c — d would no longer be bare c — d, but something more 

 like ahc — def, — palpably differing impressions. But in ac- 

 tual experience the education can take place in a much less 

 methodical way, and we learn at last to discriminate c and d 

 without any constant adhesion being contracted betAveen 



est vision). The a's will then appear single, and so probably will the b's. 

 But the now single-seeming h on slide 1 will look nearer, whilst that on 

 slide 2 will look farther than the a. But, if the diagrams are rightly drawn, 

 6 and b'" must affect 'identical' spots, spots equallj' far to the right of 

 the fovea, b in the left eye and b'" in the right eye. The same is true 

 of b' and b". Identical spots are spots whose sensations cannot possibly be 

 discriminated as such. Since in these two observations, however, they 

 give rise to such opposite perceptions of distance, and prompt such op- 

 posite tendencies to movement (since in slide 1 we converge in looking from 

 a to b, whilst in slide 2 we diverge), it follows that two processes which 

 occasion feelings quite indistinguishable to direct consciousness may never- 

 theless be each allied with disparate associates both of a sensorial and of a 

 motor kind. Cf. Bonders, Archiv f. Ophthalmologie, Bd. 13 (1867). The 

 basis of his essay is that we cannot feel on which eye any particular ele. 

 ment of a compound picture falls, but its effects on our total perception 

 differ in the two eyes. 



