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LXXIII. On Intermittent Vision. 

 By C. Y. Raman, M.A* 



ONE of the most curious and interesting of the pheno- 

 mena met with in the borderland between the physics 

 and the physiology of vision, is the occasional appearance of 

 " intermittency " seen by an observer who watches a rapidly 

 revolving object, e. g. a disk with alternate white and black 

 sectors revolving in its own plane. The phenomenon has 

 been investigated by Mr. Mallock, who, in his paper on the 

 subject i\ puts forward the somewhat startling hypothesis 

 that a slight mechanical shock to the head or body of the 

 observer produces a periodic but rapidly extinguished para- 

 lysis of the perception of sight, and that the nerves on which 

 seeing depends cannot bear more than a certain amount of 

 mechanical acceleration without loss of sensibility. Mr. 

 Mallock's conclusions have been criticised in a recent paper J 

 by Prof. Silvanus P. Thompson, who, as the result of his 

 experiments, comes to the conclusion that Mr. Mallock's 

 hypothesis is unnecessary, and adds that the explanation of 

 the phenomenon appears to be that " when the moving images 

 of the white sectors on the retina are suddenly shifted by a 

 minute displacement, they fall upon some of the rods and 

 cones which are relatively unfatigued, and which for the 

 instant are therefore of greater sensitiveness." 



I have recently had occasion to examine this subject, and 

 I find that Prof. 8. P. Thompson's suggestion that retinal 

 fatigue is the cause of the effect is apparently also untenable, 

 as it is inconsistent with the observed phenomena. There is 

 no difficulty in testing the hypothesis that the fatigue of the 

 rods and cones in the retina is the cause of the effect. If 

 the images of the white sectors on the retina are suddenly 

 moved, by some means, to positions of greater sensitiveness, 

 namely, to the portions which the dark sectors had just 

 passed over, we should, according to Prof. iS. P. Thompson, 

 expect the white sectors to flash out bright on a dark field. 

 By observing the revolving disk in a mirror which is sud- 

 denly tilted by a small amount, the necessary conditions 

 may be secured experimentally with a stationary retina, but 

 as a matter of fact the expected effect fails to manifest itself. 

 Paradoxically enough, it is by a sudden apparent displace- 

 ment of the white sectors in the opposite direction, i. e. which 

 brings them to positions on the retina already fatigued by 



* Communicated by the Author. 



t Proc. Roy. Soc. A. vol. lxxxix. p. 407. 



X Proc. Roy. Soc. A. vol. xc. p. 448. 



