Page:Proceedings of the Royal Society of London Vol 60.djvu/396

Rh 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 constituents 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. 1G7, 292.