282 Dr. S. Bid well. On Negative After-images, and 



that the suppositions which have been put forward are correct, the 

 visible flash is not contemporaneous with the physical illumination. 

 One does not begin to experience the green sensation until after the 

 green ray which excited it has been shut off. What is actually per- 

 ceived is, in fact, a positive after-image, the duration of which may be 

 considerably longer than that of the stimulus. But if a sufficiently 

 luminous white surface is presented to the eye immediately upon the 

 expiration of the brief period of stimulation by green light, the after- 

 image formed will not be positive but negative, and the only colour 

 perceived will be purple. The fatigue to which the negative image is 

 due must have been set up during the latent period when no image at 

 all was actually perceived. It is noteworthy that if the white back- 

 ground is eclipsed by black before the expiration of the period during 

 which the positive after-image normally continues, the purple negative 

 after-image is seen to be followed by a green positive one, which appears 

 as a bright object upon the dark ground. 



One other point requires notice. According to Hering's theory, rays 

 of every wave-length excite not only the sensation of a colour but 

 also that of white. Supposing therefore that the colour-sensation lags 

 behind the white-sensation, we should expect that when the zinc disc 

 is turned, the black spot, even if no colour showed upon it, would 

 appear more or less grey. This, however, is not the fact, at least to 

 any perceptible extent; on the contrary, the spot appears more 

 intensely black when it is illuminated by intermittent green light than 

 it does when the green light is screened off. In the latter case (when 

 no light whatever falls upon it) the spot seems to be veiled by a faint 

 haze, the origin of which I have traced to a phenomenon attending 

 sudden changes of illumination described in a former paper.* The 

 " black spot" phenomena are therefore not fully in accord with either 

 of the leading theories of colour-vision. 



Red and Green Borders. — The narrow red and blue-green borders 

 which appear to surround colour-patch images formed from different 

 parts of the spectrum obviously point to the excitation of funda- 

 mental red and blue-green colour sensations, the effects of the 

 excitation being sympathetically extended beyond the geometrical 

 boundaries of the images projected upon the retina. Bed borders are 

 exhibited by colour-patches formed from any mixture of spectral rays 

 which contains a considerable proportion of red; they also appear 

 around patches illuminated by the simple orange and yellow rays of 

 the spectrum (though with the latter they are feeble) and around 

 white patches. With mixtures of spectral rays from which red, 

 orange, and yellow rays are excluded, they are never seen. A blue- 

 green border, on the other hand, appears only when the green or the 

 blue of the spectrum enters into the combination, the addition of blue- 

 * See 'Boy. Soc.Proc.,' vol. 60, p. 370, experiment I (2). 



