1894.] Images following Visual Impressions 



Table of Mr. Davis's Observations. 



133 



and he concludes that when any one of the three kinds of Young- 

 Helmholtz nerve 6bres is excited, an excitation is induced in the 

 nerve fibres of the other kinds, the process being analogous to the in- 

 duction of electric currents. 



In 1885 I called attention to a very simple and effective method of 

 exhibiting a recurrent image.* If an ordinary vacuum tube, illu- 

 minated by an induction coil discharge, is made to rotate slowly 

 upon a horizontal axis fixed at right angles to the middle of the tube, 

 the tube is seen to be foJ lowed at a distance of a few degrees by a 

 ghost-like image of itself, the ghost exactly imitating the original in 

 form, but having a uniform steel-grey colour. In the same paper the 

 following observation is noted : " The vacuum tube being at rest in 

 a feebly lighted room, I concentrated my gaze upon a certain small 

 portion of it while the discharge was passing. The current was then 

 interrupted and the luminous image was almost instantly replaced by 

 a corresponding image which appeared to be intensely black upon a 

 less dark back-ground. After a period which I estimated at from a 

 quarter to half a second the black image again became luminous ; 

 this luminous impression lasted but for a small fraction of a second 

 and the series of phenomena terminated with its disappearance 



It was also found desirable to make the preliminary 



illumination as short as possible, a single flash being generally 

 sufficient to produce the phenomena." The following comment was 

 added : " The series of phenomena seem to be due to an affection of 

 the optic nerve which is of an oscillatory character. Abnormal dark- 

 ness follows as a reaction after the luminosity, and again after ab- 

 normal darkness there is a rebound into feebler luminosity." 



The subject has recently attracted much attention in connection, 

 with the experiments of M. Aug. Charpentier. The account of 

 them given by M. Charpentier in a paper on " Retinal Oscillations"! 

 is briefly as follows : If a black disk having a white sector is illu- 

 minated by a strong light, and slowly turned round while the 



* ' Nature,' vol. 32 (1885), p. 30. 



t "Oscillations retiniennes," ' Comptes Rendus,' vol. 113 (1891), p. 1.47. See 

 also " Reaction oscillatoire de la Retine," ' Arch, de Physiologic,' 1892, p. 541. 



