REFERRING to Sir Oliver Lodge's conviction that the green flash at sunset is mainly physiological (NATURE, Dec. 3, p. 807), I would point out that this does not explain the appearance at sunrise. The green flash has been seen at sunrise by numerous observers; I have myself seen it repeatedly. As seen through field glasses the green flash at sunset has no appearance of being physiological; the two ends of the last visible segment of the sun take on the green hue first, and this spreads rapidly over the whole segment as it grows smaller, until the last visible part of the sun becomes a vivid blue green. This appearance as seen through field glasses has been described more than once. The green flash is only seen when the sun sets behind a distant horizon, whether this be sea, land, or cloud, as would be expected if the flash were due to refraction. If the cause were physiological the flash should be seen when the sun sets behind a horizon close at hand, but so far as I know this is not the case.