In the Royal Society paper before referred to the point is further discussed, and a method is described by which the stage of reaction may be easily exhibited and its duration approximately measured. If a translucent disk, made of stout drawing-paper and having an open sector, is caused to rotate slowly in front of a luminous background, a narrow radial dark band, like a streak of black paint, appears upon the paper very near the edge which follows the open sector. From the space covered by this band when the disk was rotating at a known speed, the duration of the dark reaction was calculated to be about one-fiftieth of a second; my original estimate was therefore an excessive one. The experiment is illustrated in Fig. 42. One more interesting point should be noticed in the train of visual phenomena which attend a period of illumination. The sensation of luminosity which is excited when light first strikes the eye is for about a sixtieth of a second much more intense than it subsequently becomes. This is shown by the fact, which is obvious enough when once attention has been directed to it, that the bright band, which in the Charpentier disk intervenes between the dark band and the leading edge of the white sector, appears to be much more strongly illuminated than any other portion of the sector. The complete order of visual phenomena observed when the retina is exposed to the action of light for a limited time may therefore be summed up as follows:-- (1) Immediately upon the impact of the light there is experienced a sensation of luminosity, the intensity of which increases for about one-sixtieth of a second: more rapidly towards the end of that period than at first. (2) Then ensues a sudden re-action, lasting also for about one-sixtieth of a second, in virtue of which the retina becomes partially insensible to renewed or continued luminous impressions. These two effects may be repeated in a diminished degree, as often as three or four times. (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.