THE SENSES. 227 



falls on the 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 obliter- 

 ate it, but so as to cause it to become the contrast or opposite of itself. 

 Thus a grey spot upon a white ground appears darker than the same tint 

 of grey 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 grey strip of paper, lying upon an extensive surface of any 



FIG. 385.- Diagram of the axes of rotation to the eye. The thin lines indicate axes of rotation, 

 the thick the position of muscular attachment. (Modified from Fick.) 



bright color, does not appear grey, but has a faint tint of the color which 

 is the complement of that of the surrounding surface. A strip of grey 

 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 ex- 

 citing color, being wholly independent of any rays of the corresponding 

 color acting from without upon the retina, must arise as an opposite or 



