benham's artificial spectrum top. 137 



other. Sometimes the coloured bands seem mottled or the colours 

 appear in flashes radiating from the centre of revolution. 



Prof. Liveing is not at all satisfied with the explanation pub- 

 lished by Mr. Benham, which is to the effect that the average 

 number per second of stimuli affecting tlie eye is increased, or 

 diminished, according as a black portion of a band follows or pre- 

 cedes a white ground, in the revolution of the disc. 



The only explanation Prof. Liveing can himself offer is based, 

 firstly, on the known facts that the impression produced on the 

 retina by a bright object remains for an appreciable time after the 

 light from the object has been cut off, and that the duration of 

 that impression is different for different colours (a result discovered 

 by the late eminent physicist and physiologist. Von Helmholtz) ; 

 and secondly, on a supposition (which he does not know to have 

 been as yet verified experimentally) that the rapidity with which 

 the eye perceives colours is 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 black is due to the greater rapidity with which the eye 

 perceives red light than that with which it perceives blue. 



If, however, the alternations of white and black succeed each 

 other with sufficient rapidity, the new impression of a white patch 

 will be produced before that of its predecessor has vanished, and 

 there will be an overlapping of impressions, and the sensation will 

 be that of a mixture of colours of a more or less neutral tint. 

 This is in accordance with the observation that if several circular 

 bands, each partly black and partly white, in equal proportion, are 

 on the same disc ; but in some of the bands the whole of the 

 black parts is in one patch, while in others it is divided up into 

 several patches, then, when the disc is rotated so that the band 

 which presents only two alternations at each revolution is seen 

 coloured, the bands which present a greater number of alternations 

 are seen of a neutral tint. Also, if the rotation is rapid enough, 

 the bands all appear of a neutral tint. 



So far as he can test the theory by his own eyes, it appears to 



Prof. Liveing that the residual impression, left when the light from 



International Journal of Microscopy and Natural Science. 



Third Series. Vol. V. l 



