THE PERCEPTION OF SIGHT. 289 



nearer than the point at which we are looking, and are 

 not too far removed from it laterally to admit of their 

 position being sufficiently seen. At first we can only 

 recognise double images of objects at very different 

 distances from the eye, but by practice they will be seen 

 with objects at nearly the same distance. 



All these phenomena, and others like them, of double 

 images of objects seen with both eyes, may be reduced 

 to a simple rule which was laid down by Johannes 

 Miiller : — ' For each point of one retina there is on the 

 other a corresponding point.' In the ordinary flat field 

 of vision presented by the two e}es, the images received 

 by corresponding points as a rule coincide, while images 

 received by those which do not correspond do not co- 

 incide. The corresponding points in each retina (without 

 noticing slight deviations) are those which are situated 

 at the same lateral and vertical distance from the point 

 of the retina at which rays of light come to a focus when 

 we fix the eye for exact vision, namely, the yellow spot. 



The reader will remember that the intuitive theory 

 of vision of necessity assumes a complete combination 

 of those sensations which are excited by impressions 

 upon corresponding, or, as Miiller calls them, ' identical ' 

 points. This supposition was most fully expressed in 

 the anatomical hypothesis, that two nerve fibres which 

 arise from corresponding points of the two retinae actually 

 unite so as to form a single fibre, either at the commissure 

 of the optic nerves or in the brain itself. I may, how- 

 ever, remark that Johannes Miiller did not definitely 

 commit himself to this mechanical explanation, although 

 he suggested its possibility. He wished his law of iden- 

 tical points to be regarded simply as an expression of 

 facts, and only insisted that the position in the field of 

 vision of the images they receive is always the same. 



But a difficulty arose. The distinction between the 



