SENSATION. 15 



cult to isolate them and observe them iu their purity. Yet 

 it is evidently of the greatest importance to do so, if one 

 would conduct his iuvestigations accurately. Neglect of 

 this principle has led to many mistakes being made in 

 accounting for the facts observed. As -we have seen, if the 

 eye is allowed to wander here and there about the field as 

 it ordinarily does, successive contrast results and allowance 

 must be made for its presence. It can be avoided only by 

 carefully fixating with the well-rested eye a point of one 

 field, and by then observing the changes which occur in 

 this field when the contrasting field is placed by its side. 

 Such a course will insure pure simultaneous contrast. But 

 even thus it lasts in its purity for a moment only. It 

 reaches its maximum of effect immediately after the intro- 

 duction of the contrasting field, and then, if the fixation is 

 continued, it begins to weaken rapidly and soon disappears ; 

 thus undergoing changes similar to those observed when 

 any field whatever is fixated steadil}^ and the retina becomes 

 fatigued by unchanging stimuli. If one continues still 

 further to fixate the same point, the color and brightness 

 of one field tend to spread themselves over and mingle with 

 the color and brightness of the neighboring fields, thus 

 substituting * simidtaneo^is induction ' for simultaneous con- 

 trast. 



Not only must we recognize and eliminate the efi'ects of 

 successive contrast, of tem23oral changes due to fixation, 

 and of simultaneous induction, in analyzing the phenomena 

 of simultaneous contrast, but we must also take into account 

 various other influences which modify its effeets. Under favor- 

 able circumstances the contrast-effects are very striking, 

 and did they always occur as strongly they could not fail 

 to attract the attention. But they are not always clearly 

 apparent, owing to various disturbing causes which form no 

 exception to the laws of contrast, but which have a modi- 

 fying effect on its phenomena. When, for instance, the 

 ground observed has many distinguishable features — a 

 coarse grain, rough surface, intricate pattern, etc. — the con- 

 trast effect appears weaker. This does not imply that the 

 effects of contrast are absent, but merely that the resulting 

 sensations are overpowered by the many other stronger sen- 



