662 THE SENSE OF SIGHT. 



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, ex- 

 tend 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 coloured paper lying upon 

 a white surface, the image of the coloured object, 

 especially when it 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 obliterate 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, colours may be produced by con- 

 trast. Thus, a very small dull-grey strip of paper, lying 

 upon an extensive surface of any bright colour, does not 

 appear grey, but has a faint tint of the colour which is the 

 complement of that of the surrounding surface (see page 

 661). A strip of grey paper upon a green field, for 

 example, often appears to have a tint of red, and when 



