3 Mirage and similar phenomena and the aurora are common.
North-west is the Store Vildmose, a swamp where the mirage is seen in summer.
Apart then from absorption there will be a discontinuous change in brightness in the apparent disk at that value of the angular radius d which corresponds to tangential emission from the upper lever r' of this mirage-forming region.
Strobridge, In Miners' Mirage Land (Los Angeles, 1904); H.
The distribution of density is similar to that attending a desert mirage, but the transition is not so abrupt.