1894.] Prof. Liveiiig, On Benhanis Artificial Sjyectrum. 249 



November 26, 1894. 

 Professoii J. J. Thomson, President, in the Chair. 

 The following Communications were made to the Society : 



(1) On Benham's Artificial Spectrum. By Professor G. D. 

 LiVEING. 



Professor Liveing exhibited one of Benham's "Artificial Spec- 

 trum tops," and a variety of discs with figures in black disposed 

 on a white ground, and Avith white figures on a black ground, 

 which when revolved in a bright light shewed remarkable bands 

 of colour of various shades of red, green and blue. The general 

 result of his observations of these discs was that if a succession 

 of black and white objects were presented to the eye with moderate, 

 but not too great, rapidity, then, when black was followed by 

 white, an impression of a more or less red colour was perceived, 

 while when white was succeeded by black a more or less blue 

 colour was perceived. If the succession of black and white was 

 very rapid the appearance presented to the eye was of a more or 

 less neutral green or drab. 



He found that for different people the rate of rotation necessary 

 to produce the impression of a particular tint varied somewhat. 

 When excentric or spiral bands of black were used, the impression 

 on the eye was that of fringed bands, reddish on one side and 

 greenish on the other. Sometimes the coloured bands seemed 

 mottled or the colours appeared in flashes radiating from the 

 centre of revolution. 



He did not feel at all satisfied with the explanation published 

 by Mr Benham which, if he understood it, depends on a sort of 

 Doppler principle, and is to the effect that the average number 

 per second of stimuli affecting the eye is increased, or diminished, 

 according as a black portion of a band followed, or preceded, a 

 white ground, in the revolution of the disc. 



The only explanation Professor Liveing could himself offer, 

 was based on the known facts that the impression produced on 

 the retina by a bright object remained for an appreciable time 

 after the light from the object had been cut off, and that the 

 duration of that impression was different for different colours ; 

 and on a supposition, which he did not know to have been as yet 

 verified experimentally, that the rapidity with which the eye 

 perceives colours was greater for one end of the spectrum than for 

 the other. From this point of view the explanation of the blue 

 colour seen when white is followed by black would be that the 

 impression of blue on the retina lasts a little longer than that of 

 the other colours; while the red colour seen when white succeeds 



