SENSE ORGANS AND SENSATIONS 253 



the fibers of the optic nerve from the retina. Just how the 

 light falling upon the retina originates these impulses cannot 

 be discussed here ; suffice it to say that the character of the 

 impulse differs according to the color of the light 1 stimulat- 

 ing the retina ; the lens focuses upon the retina a flat, colored 

 picture of the objects at which it is looking, just as a pho- 

 tographic camera does, or as the painter represents a scene 

 on canvas. One part of the retina is thus stimulated by light 

 of one color, and another part by light of another color or 

 by another shade of the same color ; and the different kinds 

 of impulses started in the fibers of the optic nerve ultimately, 

 upon their arrival in the brain, excite in consciousness what 

 we know as visual sensations. The sensations which we get 

 from the retina are therefore primarily sensations of color. 



15. Visual judgments. But when we look at an object we 

 get more than mere sensations of color. The world does 

 not appear to us as a flat surface, of different colors, like 

 the painter's canvas. When we look at the wall of a room 

 we know that it is a flat surface, and when we look at a box 

 we know that it has not only length and breadth but also 

 thickness. If we were dependent entirely upon the retinal 

 image for our idea of the box, it would look as flat as the 

 wall ; that it does not appear so is because we receive other 

 information about the box than that which comes from the 

 retina. We have to accommodate the lens differently for 

 the near and the far edges, and we have learned by experi- 

 ence that this necessity indicates depth, or different distances 

 of different parts of the object. Again, we see the box with 

 both eyes, and the images formed on the two retinas are not 

 exactly the same. One eye sees more of one side, the other 

 eye more of another side ; and while we are not conscious 

 of this fact, we have really learned by experience and by 

 the actual handling of objects that this slight difference in 



1 In this and the following paragraphs white, black, and gray are 

 regarded as colors. 



