SINGLE VISION WITH TWO EYES 449 



the impression of the latter has died away. The result 

 is that the succession of different pictures irresistibly 

 suggests one or more objects undergoing successive 

 changes — the juggler seems to throw the balls, and the 

 boys appear to jump over one another's backs The same 

 explanation holds good for the cinematograph. (See 

 p. 428). 



10. Single Vision with Two Eyes. Corresponding 

 Points. — When an external object is asceiiained by touch 

 to be single, the centres of its retinal images in the two eyes 

 fall upon the centres of the yellow spots of the tico eyes, when 

 both eyes are directed toicards it ; but if there be two external 

 objects, the centres of both their images cannot fall, at the 

 same time, upon the centres of the yelloic spots. 



Conversely, when the centres of tioo images, formed 

 simidtaneously in the two eyes, fall upon the centres of the 

 yellow spots, the mind judges the images to be caused by a 

 single external object ; but if not, by two. 



This seems to be the only adn>issible explanation of the 

 facts, that an object which appears single to the touch 

 and when viewed with one eye, also appears single when 

 it is viewed with both eyes, though two images of it are 

 necessarily formed ; and on the other hand, that when 

 the centres of the two images of one object do not fall on 

 the centres of the yellow spots, both images are seen 

 separately, and we have double vision. In squinting, the 

 axes of the two eyes do not converge equally towards the 

 object viewed. In consequence of this, when the centre 

 of the image formed by one eye falls on the centre of the 

 yellow spot, the corresponding part of that formed by the 

 other eye does not, and double vision is the result. 



For simplicity's sake we have supposed the images to 

 fall on the centre of the yellow spot. But though vision 

 is distinct only in the yellow spot, it is not absolutely 

 limited to it ; and it is quite possible for an object to be 

 seen as a single object with two eyes, though its images 

 fall on the two retinas outside the yellow spots. AU that 



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