Further Observations on Nova Persei. 133 



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, f 



(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. 



" Further Observations on Nova Persei. No. 4." By Sir NOKMAN 

 LOCKYEII, K.C.K, F.Ii.S. Eeceived October 10, 1901. 



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. 



* ' Phil. Trans.,' B, vol. 191, p. 7. 



t I find no evidence of any such cross-connection between one colour sensation 

 and another. 



t ' Roy. Soc. Proc.,' vol. 68, p. 399. 

 VOL. LXIX. L 



