376 Mr. S. Bidwell. On Subjective Colour Phenomena 



partly by the diminished sensibility of the retina after the first 

 moment and partly by the contraction of the iris. The dark in- 

 terior 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. 



Bufc 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 con- 

 sequently 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 evi- 

 dently 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 sud- 

 denly 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 been withdrawn, this reaction being more marked in the red fibres 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 



* 'Comptes Rendus,' vol. 113 (1891), p. 147. 



