THE SENSE OF VISION. 793 



part and darker near a lighter part, and its color seen near another color 

 approaches the complementary color of the latter. A contrast phenomenon 

 similar in its effects to that above described may be produced under conditions 

 in which negative after-images can play no part. This kind of contrast is 

 known as simultaneous contrast, and may perhaps be explained on the theory 

 that a stimulation of a given portion of the retina produces in the neighboring 

 portions an effect to some extent antagonistic to that caused by direct stimulation. 



A good illustration of the phenomenon of contrast is given in Figure 245, 

 in which black squares are separated by white bands which at their points of 

 intersection appear darker than where they are bordered on either side by the 

 black squares. 



A black disk on a yellow background seen through white tissue-paper 

 appears blue, since the white paper makes the black disk look gray and the 

 yellow background pale yellow. The gray disk in contrast to the pale yellow 

 around it appears blue. 



The phenomenon of colored shadows also illustrates the principle of con- 

 trast. These may be observed whenever an object of suitable size and shape 

 is placed upon a sheet of white paper and illuminated from one direction by 

 daylight and from another by gaslight. Two shadows will be produced, one 

 of which will appear yellow, since it is illuminated only by the yellowish gas- 

 light, while the other, though illuminated by the white light of day, will 

 appear blue in contrast to the yellowish light around it. 



Space-perception. Rays of light proceeding from every point in the 

 field of vision are refracted to and stimulate a definite point on the sur- 

 face of the retina, thus furnishing us with a local sign by which we can 

 recognize the position of the point from which the light proceeds. 

 Hence the size and shape of an optical image upon the retina enable us to 

 judge of the size of the corresponding object in the same way that the cutane- 

 ous terminations of the nerves of touch enable us to judge of the size and 

 shape of an object brought in contact with the skin. This spatial perception 

 is materially aided by the muscular sense of the muscles moving the eyeball, 

 for we can obtain a much more accurate idea of the size of an object if 

 we let the eye rest in succession upon its different parts than if we gaze fixedly 

 at a given point upon its surface. The conscious effort associated with a given 

 amount of muscular motion gives, in the case of the eye, a measure of distance 

 similar to that secured by the hand when we move the fingers over the surface 

 of an object to obtain an idea of its size and shape. 



The perception of space by the retina is limited to space in two dimensions 

 i. e. in a plane perpendicular to the axis of vision. Of the third dimension 

 in space i. e. of distance from the eye the retinal image gives us no know- 

 ledge, as may be proved by the study of after-images. If an after-image of 

 any bright object e. g. a window be produced upon the retina in the man- 

 ner above described and the eye be then directed to a sheet of paper held in 

 the hand, the object will appear outlined in miniature upon the surface of the 

 paper. If, however, the eye be directed to the ceiling of the room, the object 



