TEE PERCEPTION OF SPACE. 



225 



jection of images on geometrically similar points that imagss 

 which fall upon geometrically disparate pom^s of the tivo retime 

 should be projected in disparate directions, and that tJieir objects 

 should consequently appear in two places, or look double. 

 Take the jjarallel rays from a star falling upon two eyes 

 which converge upon a near olDJect, O, instead of being 

 parallel, as in the previously instanced case. If SL and SR 

 in Fig. 55 be the parallel rays, each of them will fall upon 

 the nasal half of the retina which it strikes. 



Fig. 55. 



But the two nasal halves are disparate, geometrically 

 symmetrical, not geometrically similar. The image on the 

 left one will therefore appear as if lying in a direction left- 

 ward of the Cyclopean eye's line of sight ; the image of the 

 right one will appear far to the right of the same direction. 

 The star will, in short, be seen double, — ' homonymously ' 

 double. 



Conversely, if the star be looked at directly with parallel 

 axes, O will be seen double, because its images will affect 

 the outer or cheek halves of the two retinae, instead of one 

 outer and one nasal half. The position of the images will 

 here be reversed from that of the previous case. The right 



