THE EYE AS A SENSORY APPARATUS. 541 



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 ratina. There is no other eye behind the retina look- 

 ing at it to see the inversion of the image 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 cen- 

 tralis ; 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 any- 

 thing 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 1 in- 

 terpret the stimulation of lower retinal regions as meaning 

 high objects, and of right retinal regions as meaning left ob- 

 jects, 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, 

 except by mere chance, reach towards an object which it saw; 

 it would grasp at random, not yet having learnt that to reach 

 an object exciting a part of the retina above the fovea needed 

 movement of the hand towards a position in space below the 

 visual axis ; but very soon it learns that things near its brow, 

 that is up, excite certain visual sensations, and objects below 

 its eyes others, and similarly with regard to right and left; in 

 time it learns to interpret retinal stimuli so as to localize 

 accurately the direction, with reference to its eyes, of outer 

 objects, and never thenceforth gets puzzled by retinal inver- 

 sion. 



Color Vision. Sunlight reflected from snow gives us a 

 sensation which we call white. The same light sent through 

 a prism and reflected from a white surface excites in us, mo 



