Page:Proceedings of the Royal Society of London Vol 60.djvu/403

376 partly by the diminished sensibility of the retina after the first moment and partly by the contraction of the iris. The dark interior of the halo, which begins to appear soon after its formation, is probably connected with a class of visual sensations which have been specially studied by M. Aug. Charpentier.* The sensation of luminosity is followed very shortly after its first excitment by a brief, dark reaction, and it is perhaps the momentary revival of the luminosity after this reaction that gives the halo the appearance of retreating into the bright disk.

But whatever the cause of the .halo, there can hardly be any doubt that the corona or narrow red border is due to sympathetic excitation. When the red nerve-fibres of the Young-Helmholtz theory are affected by light the intensity of which does not exceed a certain limit, the immediately surrounding red nerve-fibres are for a short period sympathetically affected, while the violet and green are not so, or in a much less degree.

It must be confessed that it is more difficult to offer a reasonably simple explanation of what happens when the intensity of the light exceeds the limit above indicated, and the band of greenish-blue consequently appears in addition to, or in place of, the red border. It is, perhaps, preferable to refrain at present from any speculation on the subject.

When a Benham’s top is spun in bright daylight or weak sunshine, it is quite possible to distinguish both the red and the greenish-blue at the same time, the latter encroaching somewhat upon the white ground; its persistence is greater than that of the red, as can easily be seen when the top is turning rather slowly. The greenish-blue appears to be of the hue that is complementary to red, and it is evidently the development of this colour that makes the red so much less conspicuous when the top is illuminated by daylight than when artificial light is employed.

The obvious method of accounting for the formation of the blue border around a patch in a bright field from which light has suddenly been cut off, is to suppose a brief sympathetic reaction in the nerve-fibres adjacent to those from which the exciting stimulus has been withdrawn, this reaction being more marked in the red fibres than in the green and violet, or perhaps occurring in the red fibres only, at least when the light is of the usual intensity. If the red fibres just outside the darkened patch ceased for a moment to respond to the luminous stimulus, in sympathy with those inside the patch, the appearance of a blue border would be produced. In sunlight I have sometimes found that the lines in Benham’s top which ordinarily appear blue, assumed a reddish colour; under


 * 1) ‘ Comptes Rendus,’ vol. 113 (1891), p. 147