When, however, a visiting card which had been blackened over its whole surface was placed behind the rotating disk, it merely turned a lighter black, or rather grey, in which it was impossible to imagine the slightest tinge of blue.
A small piece of white paper which was subsequently attached to the middle of the card became blue around its edges when the disk was turned, but the blue did not encroach at all (or if at all, only to a very small extent) upon the black ground.
When these observations have been made it becomes possible to recognise that the apparently blue lines in the top are themselves really grey, and only bordered externally with blue. attending sudden Changes o f Illumination.
Experiment VIII.
The natural conclusion from the observations described above is that if a black disk were suddenly formed upon a bright ground, the disk would for a moment appear to be surrounded by a blue border. I was not successful in devising a satisfactory arrangement for suddenly creating a black disk, but the effect is sufficiently shown in the following manner.
An aperture l j in. (3 cm.) in diameter was cut in one side of a wooden box and was covered with white paper; one half of the aperture could be suddenly covered by a sliding metal shutter which was actuated by a spring : a lamp was placed inside the box. When the shutter was operated, a blue band 1 or 2 mm. wide appeared on the bright ground just beyond and adjoining the edge of the shutter when at rest. Its duration was thought to be slightly longer than that of the red border of other experiments, and it appearently disappeared by retreating into the black edge of the shutter.
When the shatter was moved by hand across the field at a slower speed, its edge was seen to be preceded by a thin blue border, which, when the shutter reached its limiting stop, appeared to reverse the direction of its motion and return into the shutter. The blue border is much less conspicuous and more difficult of observation than the red one. In order to see it plainly careful adjustment of the light is necessary. An examination of the effect through coloured glasses was attended by uncertain results.
Remarks on the Experiments.
The phenomenon which in the account of Experiment I has been spoken of as a blue halo may be due either to a momentary sympathetic excitement of the nerve fibres of the retina in the neighbourhood of those directly acted upon by the light, or, as I think, less probably, to light scattered by the imperfectly transparent media of the eye. In the latter case its rapid disappearance might be accounted for
