COLOUR VISION AND COLOUR-VISION THEORIES 485 



the eyes have been previously exposed to a fairly bright light. 

 An opaque disk in a spectacle frame suffices admirably, a certain 

 amount of light being allowed to enter the eye from the 

 periphery. 



The currents carry the visual quality, colour, and brightness 

 of the region from whence they come into an after-image. They 

 also tend to move an after-image towards the centre, thus if we 

 have two similar after-images, one situated in the centre and the 

 other a short distance from the centre, the one external to the 

 centre may be carried into the centre and combine with the one 

 already there. 



These currents are formed by the flow of sensitised liquid. 



Movement of Positive After-image (39, 40, 41). — I have shown 

 how the positive after-image may by a jerk with the head be 

 separated from the negative after-image and that multiple after- 

 images can be caused by one light stimulus. This shows that 

 the photo-chemical stimulus is external to the cones and can be 

 moved. 



Dark and Light Adaptation. — We have an easy explanation 

 of dark adaptation by assuming that the liquid round the cones 

 becomes more sensitive through a greater percentage of visual 

 purple being poured into it. In light adaptation the anatomical 

 arrangement is such as to prevent as far as possible the decom- 

 position of the visual purple. 



The above is only a very small portion of the evidence 

 which might have been given in support of the theory advanced. 

 The reader will find further facts in my papers in the Journal of 

 Physiology and elsewhere, and I have stated in the Journal 

 of Physiology that this theory should be accepted as the working 

 hypothesis of vision until some fact is found which is incon- 

 sistent with it. I have discussed the theory with the chief 

 workers of the world on vision and they have not been able to 

 point out any fact which is opposed to it. Should any reader 

 be able to do so, I should be glad to hear from him. 



Conclusion 



The reader might like to know my answer to any objections 

 raised to the given theories of vision and colour vision. There 

 have never been any, and it is therefore difficult to know how to 

 deal with opponents who only stab in the back and will not 



