THE SENSES. 737 



live points of that membrane, have a certain influence on each other; 

 the particular condition of one influencing the other, so that the image 

 perceived by one part is modified by the image depicted in the other. 

 The phenomena which result from this relation between the different 

 parts of the retina, may be arranged in two classes: the one including 

 those where the condition existing in the greater extent of the retina is 

 imparted to the remainder of that membrane; the other, consisting of 

 those in which the condition of the larger portion of the retina excites, 

 in the less extensive portion, the opposite condition. 



1. When two opposite impressions occur in contiguous parts of an 

 image on the retina, the one impression is, under certain circumstances, 

 modified by the other. If the impressions occupy each one-half of the 

 image, this does not take place ; for in that case, their actions are equally 

 balanced. But if one of the impressions occupies only a small part of 

 the retina, and the other the greater part of its surface, the latter 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 ob- 

 literate it, but so as to cause it to become 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 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 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 

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