The author remarked that one of the constituents of the double fog-bow described in some of Mr Omond's recent observations, is obviously the ordinary primary rainbow, diminished in consequence of the very small size of the water drops. But the other, having nearly the same radius but with its colours in the opposite order, appears to be due to ice-crystals in the fog. This is quite consistent with the record of temperatures. Just as small drops of water may remain unfrozen in air below 0° C, small ice crystals may remain unmelted at temperatures above that point.