(3) The stage of fluctuation is succeeded by a sensation of steady luminosity, the intensity of which is, however, considerably below the mean of that experienced during the first one-sixtieth of a second.
(4) After the external light has been shut off, a sensation of diminishing luminosity continues for a short time, and is succeeded by a brief interval of darkness.
(5) Then follows a sudden and clearly-defined sensation of what may be called abnormal darkness—darker than common darkness—which lasts for about one-sixtieth of a second, and is followed by another interval of ordinary darkness.
(6) Finally, in about a fifth of a second after the extinction of the external light, there occurs another transient impression of luminosity, generally violet coloured, after which the uniformity of the darkness remains undisturbed.
[Fig. 43], which is copied from my paper, gives a rough diagrammatic representation of the above described chain of sensations. No account is here taken of the comparatively feeble after-images which succeed the recurrent image, and may last for several seconds.
I propose now to say a few words about a curious phenomenon of vision which a short time ago excited considerable interest.
Fig. 43.—Visual Sensations attending a period of Illumination.