‘The Green Flash’

Abstract

MUCH confusion has beset this subject, because two entirely different things are unfortunately covered by the phrase ‘green flash’. If separate names had from the first been given to the two things, each would have been treated apart from the other and a great amount of controversy avoided. A greenish light appears as a physiological effect on ceasing to look at the red disc of the setting sun, as Sir Oliver Lodge points out in NATURE of Dec. 3. But there is also a purely physical phenomenon of sunset which is seen at the instant when the last portion of the sun's disc disappears on a sharp horizon. During the years when I was much at sea I used to watch every clear sunset, and rarely failed to observe this phenomenon though it was sometimes incomplete. It appeared at its best on half-a-dozen occasions when the thinnest segment of the sun's disc was still above the horizon, but with its light so much reduced that it could be looked at easily through a field-glass. However red the sun may have been when fully visible, the tiny shaving of a flat arc had become distinctly yellow, and as it was disappearing, the ends turned greenish and seemed to shrink towards the centre, at which the last light visible seemed to be intensified to a clear green point, which changed into blue and vanished in violet in a fraction of a second too short to estimate.