518 THE HUMAN BODY. 



its highest power. Elsewhere on the retina our discrimi- 

 nating power is much less and diminishes as the distance 

 from the yellow spot increases. This is partly due, no doubt, 

 to a less sensibility of those retinal regions, such as, by other 

 facts, is proved to exist, but in part no doubt is also due to 

 a want of practice. The more peripheral the retinal region 

 the less we have used it for such purposes. It is probable, 

 therefore, that outlying portions of the retina are capable 

 of education to a higher discriminating power, just as we 

 shall find the skin to be for tactile stimuli. 



While we can tell the stimulation of an upper part of 

 the retina from a lower, or a right region from a left, it 

 must be borne in mind that we have no direct knowledge 

 of which is upper or lower or right or left in the ocular 

 image. All our visual sensations tell us is that they are 

 aroused at different points, and nothing at all about the 

 actual positions of these on the retina. There is no other 

 eye behind the retina looking at it to see the inversion of 

 the image (p. 496) formed on it. Suppose I am looking at 

 a pane in a second-story window of a distant house: its 

 image will then fall on the fovea centralis; the line joining 

 this with the pane is called the visual axis. The image of 

 the roof will be formed on a part of the retina below the 

 fovea, and that of the front door above it. I distinguish 

 that the images of all these fall on different parts of the 

 retina in certain relative positions, and have learnt, by the 

 experience of all my life, that when the image of anything 

 arouses the sensation due to excitation of part of the 

 retina below the fovea the object is above my visual axis, 

 and vice versa; similarly with right and left. Consequently 

 I interpret the stimulation of lower retinal regions as mean- 

 ing high objects, and of right retinal regions as meaning left 

 objects, and never get confused by the inverted retinal 

 image about which directly I know nothing. A new-born 

 child, even supposing it could use its muscles perfectly, 

 could not seize a reachable object which it saw; it would 

 not yet have learnt that attaining a point exciting that part 

 of the retina above the fovea, meant reaching a position in 

 space below the visual axis; but very soon it learns that things 



