570 PHYSIOLOGY 



tion of after images is so extensive that they begin to obtrude more 

 and more on the impression conveyed to consciousness. 



FATE OF AFTER IMAGES. If the conclusion drawn from the 

 above experiments is valid, the question arises as to the apparent unimpor- 

 tance of the after image in ordinary vision. The answer is to be obtained 

 from experiments like the following : If fixation be continued until the 

 images formed on the retina appear in outline as in the previous experiments. 

 and the gaze be then quickly turned to a second fixation mark placed some 

 distance from the first, it is found that, on returning to the first mark, 

 some time has to elapse before the appearance in outline is obtained 

 again ; in fact the time taken is not very different to that required to reach 

 this stage at the beginning of a new experiment. The second impression 

 had effected an almost complete removal of the after image of the 

 first, so that on returning to the first again, the slate had as it were been 

 wiped clean, and the first impression therefore acted as if it were a new one. 

 This conclusion is entirely in agreement with our previous conclusions with 

 regard to the after image, namely that it corresponds to a period in which 

 a stimulus, similar to that to which the after image belongs, is inhibited, while 

 that of a different kind is favoured. If therefore during fixation the gaze 

 be directed elsewhere momentarily, the after image that had been 

 set up is quenched by the new impression, and on returning the 

 gaze to the fixation point the old image behaves almost like a new one. 

 The non-intrusion of the after image in ordinary vision is therefore due to 

 a considerable extent to the continual and rapid replacement of one impres- 

 sion by another, by the shifting of the gaze, and also to the spreading of the 

 accumulation of partially effaced after images more or less uniformly over 

 the retina. It has been suggested that impulses may be originated from the 

 external eye muscles on movement, which on reaching the brain assist in 

 the removal of the after images of previous stimulation. 



ADAPTATION. If the eye after being in the dark is rapidly 

 removed to the light, at first the sight is confused and the eye 

 dazzled in spite of the powerful constriction of the pupils. The 

 eye however very quickly becomes accustomed to the greater in- 

 tensity, or as we say. it becomes light adapted. In a similar manner 

 on entering the dark from the light the eye can at first see nothing, 

 but by degrees it becomes accustomed to the new conditions and ol.je< -is 

 begin to be recognised ; the eye has therefore become dark adapted. In 

 the first case the initial light stimulus reduced the sensitive HCSN of 

 the eye to light to such an extent that the eye ceased to react to an excessive 

 degree as it had done at first. Dark adaptation appears to eonsi>t of two 

 separate processes : (1) the removal of after images from the cone light- 

 receiving mechanism, and (2) the replacement of visual purple for the rod 

 apparatus ; the former predominating at high intensities and the latter at 

 low. 



FATIGUE OF THE RETINA. If the eye has been exposed to a 

 very bright light for a considerable time, there is at first inability 



