520 THE SENSE OF SIGHT. 



lateral parts of the retina, will gradually disappear, and the 

 white surface be seen in its place. 



2. In the second class of phenomena, the affection of one 

 part of the retina influences that of another part, not in 

 such a manner as to obliterate it, but so as to cause it to be- 

 come the contrast or opposite of itself. Thus a gray spot upon a 

 white ground appears darker than the same tint of gray would 

 do if it alone occupied the whole field of vision, and a shadow 

 is always rendered deeper when the light which gives rise to it 

 becomes more intense, owing to the greater contrast. The 

 former phenomena ensue gradually, and only after the images 

 have been long fixed on the retina; the latter are instantaneous 

 in their production, and are permanent. 



In the same way, also, colors may be produced by contrast. 

 Thus, a very small dull-gray strip of paper, lying upon an ex- 

 tensive surface of any bright color, does not appear gray, but 

 has a faint tint of the color which is the complement of that 

 of the surrounding surface (seepage 519). A strip of gray 

 paper upon a green field, for example, often appears to have 

 a tint of red, and when lying upon a red surface, a greenish 

 tint ; it has an orange-colored tint upon a bright blue surface, 

 and a bluish tint upon an orange-colored surface ; a yellowish 

 color upon a bright violet, and a violet tint upon a bright 

 yellow surface. The color excited thus, as a contrast to the 

 exciting color, being wholly independent of any rays of the 

 corresponding color acting from without upon the retina, must 

 arise as an opposite or antagonistic condition of that mem- 

 brane ; and the opposite conditions of which the retina thus 

 becomes the subject would seem to balance each other by their 

 reciprocal reaction. A necessary condition for the production 

 of the contrasted colors is, that the part of the retina in which 

 the new color is to be excited, shall be in a state of compara- 

 tive repose ; hence the small object itself must be gray. A 

 second condition is, that the color of the surrounding surface 

 shall be very bright, that is, it shall contain much white light. 



The retina corresponding to the point of entrance of the 

 optic nerve is completely insensible to the impressions of light. 

 The phenomenon itself is very readily shown. If we direct 

 one eye, the other being closed, upon a point at such a distance 

 to the side of any object, that the image of the latter must 



fall upon the retina at the point of entrance of the optic nerve, 

 this image is lost either instantaneously, or very soon. If, for 



