16 PSYCHOLOGY. 



sations which entirely occupy the attention. On such a 

 ground a faint negative after-image — undoubtedly due to 

 retinal modifications — may become invisible ; and even 

 weak objective differences in color may become imper- 

 ceptible. For example, a faint spot or grease-stain on 

 woollen cloth, easily seen at a distance, when the fibres are 

 not distinguishable, disappears when closer examination 

 reveals the intricate nature of the surface. 



Another frequent cause of the apparent absence of con- 

 trast is the presence of narrow dark intermediate fields, such 

 as are formed by bordering a field ivith black lines, or by the 

 shaded contours of objects. When such fields interfere with 

 the contrast, it is because black and white can absorb much 

 color without themselves becoming clearly colored ; and 

 because such lines separate other fields too far for them to 

 distinctly influence one another. Even weak objective 

 differences in color may be made imperceptible by such 

 means. 



A third case where contrast does not clearly appear is 

 where the color of the contrasting fields is too iveak or too in- 

 tense, or where there is miich difference in brightness betiveen tJie 

 tivo fields. In the latter case, as can easily be shown, it is 

 the contrast of brightness which interferes with the color- 

 contrast and makes it imperceptible. For this reason con- 

 trast shows best between fields of about equal brightness. 

 But the intensity of the color must not be too great, for then 

 its very darkness necessitates a dark contrasting field which 

 is too absorbent of induced color to allow the contrast to 

 appear strongly. The case is similar if the fields are too 

 light. 



To obtain the best contrast-effects, therefore, the contrasting 

 fields should be near together, should not be separated by shadoius 

 or black lines, should be of homogeneous texture, and should be of 

 about equal brightness and meditim intensity of color. Such 

 conditions do not often occur naturally, the disturbing in- 

 fluences being present in case of almost all ordinary objects, 

 thus making the effects of contrast far less evident. To 

 eliminate these disturbances and to produce the conditions 

 most favorable for the appearance of good contrast-effects, 



