444 ELEMKNTARY PHYSIOLOGY less. 



the retina at the end of the visual axis, i.e. a line joining 

 the object and the retina and traversing a particular region 

 of the centre of the eye. Conversely, 'when a p trt of the 

 retina is excited, by whatever means, the sensation is 

 referred by the mind to some cause outside the body in the 

 direction of the visual axis. 



When we look at an external object which is felt by the 

 touch to be in a given place, the image of the object falls 

 upon a certain part of the retina. Conversely, when a 

 part of the retina is excited, by whatever means, the sensa- 

 tion is referred by the mind to some cause outside the body 

 occupying such a position that its image would fall on that 

 part. 



It is for this reason that when a phosphene is created 

 by pressure, say on the outer and lower side of the eye- 

 ball, the luminous image appears to lie above, and to the 

 inner side of, the eye. Any external object which could 

 produce the sense of liglit in the part of the retina pressed 

 upon nmst, owing to the inversion of, the retinal images 

 (see p. 408), in fact occupy this position ; and hence the 

 mind refers the light seen to an object in that position. 



The same kind of explanation is applicable to the 

 apparent paradox that, while all the pictures of external 

 objects are certainly inverted on the retina by the refract- 

 ing media of the eye, we nevertheless see them upright. 

 It is difficult to understand this, until one reflects that 

 the retina has, in itself, no means of indicating to the 

 mind which of its parts lies at the top, and which at the 

 bottom ; and that the mind learns to call an impression 

 on the retina high or low, right or left, simply on account 

 of the association of such an impression with certain 

 coincident tactile impressions. In other words, when one 

 part of the retina is affected, the object causing the affec- 

 tion is found to be near the right hand ; when anotbei, 

 the left ; when another, the hand has to be raised to 

 reach the object ; when yet another, it has to be depressed 

 to reach it. And thus the several impressions on the 



