Page:Proceedings of the Royal Society of London Vol 69.djvu/141

Rh highly magnified spectrum is gazed at steadily in the neighbourhood of D, or F, or G, the colours begin to change and flicker exactly as if some controlling mechanism were getting fatigued. But if either of the contending sensations is thoroughly exhausted, as in artificial colour-blindness, this flickering ceases, and is abolished until the retina recovers. I have called attention to the fact that the positive after-effect or dazzle-tint of artificial colour-blindness does not undergo those cyclic changes of colour which have been so long familiar in ordinary after-images.

To sum up —

(1.) There appears to be strong evidence of the existence in the retina, between the structures in which the visual impulses originate and the beginnings of the optic nerve, of a set of structures by which the intensity of the visual impulses transmitted to the central organ is regulated.

(2.) That these structures are sensitive to light, and when excited block wholly or in part visual impulses coming from the periphery.

(3.) That these structures are cross-connected so that the excitation of one affects those in its neighbourhood, thus increasing the contrast between a bright object and its surroundings, and probably preventing the blurring of an image by diffused light from the retina,

(4.) That they may by sudden stimuli be excited to a very powerful or even a multiple response.

(5.) That many phenomena of contrast and of areal induction may be accounted for on such a hypothesis.

(6.) That Dr. Shelford Bidwell's experiments are explicable without assuming a white sensation.

The last paper contained an account of the observations of the Nova made at Kensington between March 25 and' May 7. The observations are, in the present paper, brought up to September 30. Between June 18 and August 8 no work was done on the Nova, owing chiefly to the interference of buildings and twilight.