THE SUBJECTIVE PHENOMENA OF VISION . 569 



PERIODIC STIMULI. We have seen that, if a stimulus falls dur- 

 ing the after image of a previous one, its character is altered. If the two 

 stimuli are similar the second tends to be suppressed, but if dissimilar 

 the second appears to be increased. If on the other hand the second stimulus 

 falls either before or after the after image, it appears to be unaffected. But 

 the experiments on flicker show us more than this, because even in a case 

 where a second stimulus comes before the after image of the first, it is clear 

 that the third would at a certain speed coincide with it and would therefore 

 be modified. Experiment seems to show no evidence for such an effect, 

 and we must therefore conclude that the occurrence of the second stimulus 

 in some way inhibits the after image of the first, so that its effects are not 

 apparent. Further evidence for this view is to be obtained from continuous 

 stimuli, for we do not find a sudden diminution in the intensity of the response 

 a moment or two after a continuous stimulus has begun, such as we should 

 expect if the after image of the commencement of stimulus were suddenly 

 after a short interval to assert itself. What happens to these suppressed 

 after images ; are they entirely destroyed, or are they caused to accumulate 

 until the end of the stimulus ? The evidence appears to be in favour of the 

 latter view, because an after image has a more definite character the longer 

 the stimulus. Moreover if the gaze be directed towards a fixation point, and 

 the inclination to blink be rigidly suppressed, after a few seconds the images 

 of objects which fall on the periphery of the retina begin to appear milky, 

 particularly in the shadows. At the same time the brightness of the high 

 lights seems to be reduced so that it approximates more and more closely 

 with the milkiness of the shadows. When this stage is reached objects appear 

 in outline, the contours being produced and renewed by imperfect fixation. 

 If fixation can be retained for a short period, it will be found that the 

 whole field becomes blank with the exception of the fixation mark. If this 

 also disappears momentarily, then fixation is lost, the eye makes an involun- 

 tary movement and the whole field immediately fills with detail again. In 

 this experiment, two processes seem to be going on : firstly, in the shadows, 

 the disappearance of the after images of previous impressions, the replace- 

 ment of the visual purple previously bleached, and possibly also the recovery 

 of these parts of the retina from the effects of previous stimulation, all of. 

 which will increase the sensitiveness of the retina so that it now responds 

 to the light reflected by the shadows ; secondly, in the high lights, the accumu- 

 lation of after images, the bleaching of the visual purple, and possibly the 

 effects of fatigue, all of which tend to reduce the intensity of the 

 impression. So that these processes, tending to increase the brightness 

 in the shadows and to decrease that of the high lights, finally brings them to 

 the same level. In these processes the accumulation and removal of after 

 images would appear to take a considerable part. The conclusion to which 

 we are forced is that at the beginning of a continuous stimulus the after images 

 are effectively removed until such time as the stimulus shall cease, when 

 they can be permitted to assert themselves. But if the stimulus be pro- 

 longed the suppression becomes more and more difficult, until the accumula- 



