attending sudden Changes of Illumination. 



369 



Benham's Top. 



lines appears, to my vision, to become bright red, the next group 

 pinkish-brown, the next a dilute olive-green, and the outer group 

 dark blue. If the direction of rotation is reversed, the order of the 

 colours is also reversed. 



By far the most striking of these several hues is the first named ; 

 hardly any one has the slightest hesitation in pronouncing it to be 

 bright red. As to the blue, there is very rarely any difference of 

 opinion, though it has sometimes been called bluish-green. The 

 hues of the two intermediate groups are much more undecided and 

 difficult to specify, especially when they are seen separately. 



The only serious attempts that I know of to explain the origin of 

 the colours shown by the top are those of Professor Liveing and of 

 Captain Abney.* Professor Liveing's explanation is based upon the 

 two hypotheses that the eye perceives certain of the coloured con- 

 stituents of white light more quickly than others, red being the first 

 to show itself, and that the duration of the impressions due to the 

 different constituents also differs, blue being the last to disappear. 

 Captain Abney thinks that the results would be sufficiently accounted 

 for if the order of persistence of the three colour sensations were 

 violet, green, and red. 



Several objections might be urged against these explanations, but 

 the adequacy of either of them seems to be conclusively negatived by 



* ' Nature,' vol. 51, pp. 167, 292. 



