48o SCIENCE PROGRESS 



an artificial light are with difficulty discriminated from white by 

 this light. 



7. Colour adaptation may bring two colours below the 

 threshold of discrimination so that the two appear exactly 

 alike, although by another kind of light a difference is plainly 

 visible. 



8. Colour adaptation increases the perception of relative 

 difference for colours other than the dominant. 



9. The conscious judgment has very little effect in colour 

 adaptation. 



10. Colour adaptation greatly helps in the correct discrimina- 

 tion of colours, and masks the effects of the very great physical 

 differences which are found in different kinds of illumination. 



11. Spectral yellow, after colour adaptation to green, still 

 appears yellow and not red. 



12. Colour adaptation appears to produce its effects by sub- 

 traction of the dominant colour sensation, and not by directly 

 increasing the complementary. Spectral blue does not appear 

 brighter after colour adaptation to yellow. 



VIII. After-images (26, 27, 38, 42) 



As in all experiments with colour pure spectral light must be 

 employed. If a monochromatic region be isolated in my spectro- 

 meter a negative after-image can be produced by looking at this 

 fixedly with one eye for twenty seconds. If the eyes be kept in 

 a vertical position, that is, one over the other, then, on the eyes 

 resuming their normal position, the after-image can be projected 

 upon the middle of a horizontal spectrum thrown upon a 

 screen. As the eye is kept rigidly fixed during the fatiguing 

 process a very clear-cut negative after-image is produced which, 

 when thrown on the screen spectrum, enables close comparison 

 to be made with adjacent parts. The stability of the after-image 

 is remarkable ; it does not change colour, and is not influenced 

 by subsequent light falling on the retina when this is not of 

 too great intensity. The after-image is in every case darker 

 than any dark object on which it is projected. 



If the portion of brain having the function of the perception 

 of colour be continually receiving impulses which, affecting it 

 and the visual centre, cause the sensation of light which is seen 

 in the absence of all light stimulation, and the whole retino- 



