
1905. | On Colour-Vision by Very Weak Light. 213 
Then it returns, and again breaks up, generally leaving an island about the size of 
the yellow spot, which lasts two or three seconds longer than the rest. The colour 
of these clouds is a rich pure violet like that of the calcium lines H and K in the are 
spectrum. ‘This phenomenon may go on from 10 minutes to half-an-hour, the violet 
patches getting smaller and appearing at longer intervals till they die away. When 
that has happened no more “ dazzle-tints” appear, and in my own case there is no 
longer any interval whatever “ between the absolute and chromatic thresholds.” 
In other words, green is distinct from blue, and blue from violet, and 
violet from red as soon as either is strong enough to produce any sensation 
at all. 
I desire to emphasize this conclusion. It is absolutely opposed to the 
statements of Hering,* Hillebrand,f Aubert, Charpentier,t Landolt,§ Vogel || 
and others, whose experiments have been regarded as classical. But it is the 
result of experiments made in the first instance with a mental bias in favour 
of the view I am opposing, and finally repeated with the improved appliances 
now at my disposal. 
The times given above represent the normal duration of the various stages 
in the fading out of the after-effects of light upon the eye. 
They are seldom shorter, but occasionally much protracted. 
Sometimes the red “dazzle-tints” return after more than half-an-hour, and the 
green persist correspondingly longer. In 1895 several times I waited for nearly 
three hours without getting beyond the green. In January of the present 
year, on one occasion the green “ dazzle-tint” persisted for 75 minutes instead 
of being quite gone at 40 minutes. I therefore discontinued the experiment 
because it would probably have involved waiting about four hours for the 
violet to pass off. 
After-effects last longer in summer than in winter, perhaps partly 
because of the greater brightness of the daylight and its longer duration. 
As a rule [ find them more persistent during ill health, but on one 
occasion, after anzsthetisation of the eyeball by cocaine, I saw the violet 
clouds one hour after the bandage had been put on, z.., one-half the usual 
interval. | 
In view of this great variation in the time required for the subsidence of 
after-effects, it is quite conceivable that with some persons they may persist 
* Hering, “ Untersuchung eines total Farbenblinden,” ‘ Pfitiger’s Archiv,’ vol. 49, 
pp. 563 to 609. ; 
+ Hillebrand, “Specifische Helligkeit der Farben,” ‘Wien Akad. Sitzber.,’ 1889, p. 70. 
t Charpentier, ‘Thése de Doctorat,’ 1877 ; also ‘La Lumiere et les Couleurs.’ 
§ Landolt, ‘Comptes Rendus,’ vol. 86; p. 495, 1878. 
|| Vogel, ‘Annalen d. Physik u. Chemie,’ 1895, vol. 54, p. 745. 
