618 HANDBOOK OF PHYSIOLOGY. 



ter may, if long continued, extend its influence over the whole retina, 

 so that the opposite less extensive impression is no longer perceived, and 

 its place becomes occupied by the same sensation as the rest of the field 

 of vision. Thus, if we fix the eye for some time upon a strip of colored 

 paper lying upon a white surface, the image of the colored object, espe- 

 cially when it falls on the lateral parts of the retina, will gradually dis- 

 appear, 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 oblit- 

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

 Thus a gray spot upon a white ground appears darker than the same 



FIG. 416. Diagram of the axes of rotation to the eye. The thin lines indicate axes of rotation, 

 the thick the position of muscular attachment. 



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 extensive 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. 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 



