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.
Strobridge, In Miners' Mirage Land (Los Angeles, 1904); H.
Another type of mirage, frequently observed at sea in the northern latitudes, is presented in the appearance of ships and icebergs as if inverted and suspended in the clouds.
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.