DISCRIMINATION AND COMPARISON. 619 



then move about a second object in the neighborhood of tlie blind spot, 

 striving meanwhile to a^<e?/fZ to this latter without moving the direction 

 of our gaze from the first object. This runs counter to all our habits, and 

 is therefore a difficult thing to accomplish. With some people it is even 

 an impossibility. But only when it is accomplished do we see the 

 second object vanish and convince ourselves of the existence of this 

 gap. 



"Finally, let me refer to the double images of ordinary binocular 

 Tision. Whenever we look at a point with both eyes, all objects on this 

 side of it or beyond it appear double. It takes but a moderate effort of 

 observation to ascertain this fact ; and from this we may conclude that 

 we have been seeing the far greater part of the external world double 

 all our lives, although numbers of persons are unaware of it, and are 

 in the highest degree astonished when it is brought to their attention. 

 As a matter of fact, we never have seen in this double fashion any 

 particular object upon which our attention was directed at the time ; 

 for upon such objects we always converge both eyes. In the habitual 

 use of our eyes, our attention is always withdrawn from such objects 

 as give us double images at the time; this is the reason why we so 

 seldom learn that these images exist. In order to find them we must 

 set our attention a new and unusual task ; we must make it explore 

 the lateral parts of the iield of vision, not, as usual, to find what objects 

 are there, but to analyze our sensations. Then only do we notice this 

 phenomenon.* 



" The same difficulty which is found in the observation of subjective 

 sensations to which no external object corresponds is found also in the 

 analysis of compound sensations which correspond to a single object. 

 Of this sort are many of our sensations of sound. AVhen the sound of 

 a violin, no matter how often we hear it, excites over and over again 

 in our ear the same sum of partial tones, the result is that our feeling 

 of this sum of tones ends by becoming for our mind a mere sign for the 

 voice of the violin. Another combination of partial tones becomes the 

 sensible sign of the voice of a clarionet, etc. And the oftener any such 

 combination is heard, the more accustomed we grow to perceiving it as 

 an integral total, and the harder it becomes to analyze it by immediate 

 observation. I believe that this is one of the principal reasons why 

 the analysis of the notes of the human voice in singing is relatively so 



* When a person squints, double images are formed in the centre of the 

 field. As a matter of fact, most squinters are found bliud of one eye, or 

 almost so ; and it has long been supposed amongst ophthalmologists that 

 the blindness is a secondary affection superinduced by the voluntary sup- 

 pression of one of tlie sets of double images, in other words by the positive 

 and persistent refusal to use one of the eyes. This explanation of the 

 blindness has, however, been called in question of late years. See, for a 

 brief account of the matter, O. F. Wadsworth in Boston Med. and Surg. 

 Journ., cxvi. 49 (Jan. 20, '87), and the replies by Derby and others a little 

 later.— W. J. 



