After-images following brief Retinal Excitation. 271 



Experiment IV. 



What may be regarded as the negative after-image of a black spot 

 upon a white ground is shown by the ordinary method of observation 

 as a patch of more intense whiteness. It was of interest to ascertain 

 whether, by means of the disk arrangement, a black object conld be 

 made to assume a continuous appearance of abnormal whiteness. 



Several broad bands | inch wide were painted with " artist's 

 black " upon a white card. When this was placed behind the 

 rotating disk and exposed to direct sunshine, the effect was ex- 

 ceedingly striking, the painted portions appearing as bands of glit- 

 tering whiteness upon a ground which by contrast seemed to be of a 

 neutral grey tint. With strong lamplight or diffused daylight the 

 phenomenon was almost equally remarkable ; but, for some unex- 

 plained reason, the ground seemed to acquire a very pale purple tint 

 instead of being, as in the former case, quite neutral. 



All the above observations can, of course, be accounted for quite 

 easily in the ordinary way by the Youug-Helmholtz theory of vision. 

 The only novelty in connection with them consists in the demon- 

 stration of the fact that the action of light after a period of darkness 

 {which need not exceed one-twentieth of a second) can appreciably 

 diminish the sensibility of the retinal nerve-fibres in a space of time 

 so extremely short that if the light be coloured its colour is not 

 consciously perceived. 



The experiments clearly indicate the origin of the greenish-blue 

 borders which I was unable to explain at the time when my 

 former paper was written. If the hypothesis suggested in that 

 paper is correct, it now seems that the coloured borders produced 

 by sudden changes of illumination are, whatever their hue, in all 

 cases primarily due to sympathetic action of the red nerve-fibres 

 of the retina. 



IS T ote added May 6. 



[The transient blue border which ordinarily appears to surround a 

 black patch suddenly formed upon a white ground (loc. cit., p. 375) 

 becomes transformed under strong illumination into a pale red one. 

 White letters printed upon black paper, when seen through the disk 

 turning the reverse way, appear to be blue under ordinary illumina- 

 tion, and red wh^n the lamp is brought very near the disk. — May 6.] 



