134 Mr. S. Bidwell. On Ike Recurrent [June 7, 



observer's eye is fixed upon its centre, there appears upon the white 

 sector, near to its leading edge, a well-defined dark band, which is 

 separated from the black ground of the disk by a similar white band. 

 The angular extension of the dark band increases with the speed of 

 rotation, so that it always takes the same time to pass over a fixed 

 point on the retina ; it begins about one-sixty-fifth or one-seventieth 

 of a second after the first passage of the white, and lasts sensibly the 

 same time. He goes on : " The dark band is in fact only a kind of 

 reaction of the retina after the luminous excitation, a reaction which 

 can be demonstrated in a totally different manner. I have found 

 that if an instantaneous luminous excitation is produced in complete 

 darkness the sensation appears to be reduplicated ; shortly after its 

 first generation it seems to disappear and then manifest itself again. 

 This is the case, for example, when a single discharge from a 

 Ruhmkorff coil is passed through a Crookes or Geissler vacuum tube, 



or simply, but less obviously, through the air There is, then, 



in this last experiment, as in the first, a negative reaction of the retina 



under the influence of excitation It would be difficult, and in 



any case premature, to indicate the cause of this phenomenon, but it 

 may fairly be characterised as the result of a retinal oscillation set up 

 under the influence of the beginning of the luminous excitation." I 

 think it clearly appears from the above extract that M. Charpentier 

 was unacquainted with the earlier observations of myself and others. 

 In consequence of the importance which seemed to be attached by 

 physiologists to the phenomena of visual reaction, as evidenced by 

 Professor Burdon Sanderson's recent Presidential Address to the 

 British Association,* I was induced to undertake the farther experi- 

 mental investigation, of which an account is given in the present 

 paper. This deals partly with the colours of recurrent images under 

 different conditions, and partly with the reaction attending the earl}* 

 stages of a luminous impression as noticed by Charpentier. 



In the observation of the recurrent images set up by the action of 

 light of different colours I began, like Mr. Davis, by using coloured 

 glasses. 



A metal disk, about 8 cm. in diameter, was arranged so as to rotate 

 slowly and steadily about its centre in front of the condenser of a 

 projection lantern. Near the edge of the disk was a circular aperture 

 about 0'5 cm. in diameter, the image of which was focussed upon a 

 distant screen. A plate of coloured glass was placed before the pro- 

 jecting lens, and thus was obtained a small, coloured disk of light, 

 which described a circular path upon the screen. The coloured disk 

 was, in most cases, seen to be followed at 8,n interval of a few degrees 

 by a ghost of the same size and shape, but of feebler luminosity, and 

 of a hue which varied more or less with the colour of the glass 

 * ' Brit. Assoc. Rep.,' 1893. ' Nature,' vol. 48, p. 468. 



