their Relation to certain other Visual Phenomena. 281 



flashed upon a white screen for about a fortieth of a second, and is 

 immediately replaced by a concentric annulus of white light. During 

 this process no green is seen at all ; there appears only a purple annulus 

 surrounding an area which is perfectly black. The white light clearly 

 has the effect of restraining the visual sense-organs adjacent to those 

 upon which it falls from responding to the green stimulus. It would 

 seem to follow a fortiori that the sense-organs directly acted upon by 

 the white light must be similarly incapacitated from evoking any green 

 sensation. It is not the fact that the green sensation is produced for a 

 moment and then swamped by a more powerful white one so completely 

 as to escape notice ; it actually never comes into existence. Neverthe- 

 less, the effects of fatigue by green are exhibited, and the physically 

 white annulus is seen as purple. 



It may be well to state that when once the necessary apparatus has 

 been set up and the various luminosities adjusted to the order of those 

 specified, the "black spot" observation is an exceedingly easy one. 

 No skilled observer is required for it ; it can be made at once by any 

 one whose vision is normal, and the phenomenon can at any time be 

 exhibited with certainty. 



No explanation of it can, I think, be afforded by the Young- 

 Helmholtz theory of colour-vision in its current form • an independent 

 white sensation must be postulated, as by the theory of Hering. 

 And the observations point to the conclusion, even if they do not of 

 themselves sufficiently prove it, that the latent period for a colour- 

 sensation is very much greater than that for white. For green, under 

 the conditions of my experiment, the latent period must be at least 

 1/40 second, while for white it can hardly exceed 1/500 second, 

 though the luminosity of the two may be nearly equal. The latent 

 period for red is probably not very different from that for green under 

 similar circumstances, that for blue being considerably greater ;* but it 

 is not quite certain whether the red and blue flickers seen upon the 

 black spot are produced before or after the illumination by white 

 light. I am inclined to think that the latter is the case, the negative 

 after-image being followed during the period of darkness by a positive 

 one. In all cases the duration of the latent period probably depends 

 partly, through certainly not wholly, upon the intensity of the 

 illumination.! 



If in a darkened room a ray of green light is admitted to the eye 

 for a period of 1/40 second, one sees a flash of green; but assuming 



* Some preliminary observations by a method of which I hope to submit an 

 account at a future date indicated that, under the conditions of the experiments, 

 the latent period was for red 0'031 sec, for green 0*028 sec, and for blue 0040 sec 



f According to Exner, " If the intensities of the illumination of an object 

 increase in geometrical progression, the times necessary for the perception of 

 the same decrease in arithmetical progression," ' Wien. Akad. Sitzber.,' vol. 58, 

 Abtheil II, p. 624, 1868. 



