E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Bifrost,
in Scandinavian mythology, is the name of the bridge between heaven and earth; the rainbow may be considered to be this bridge, and its various colours are the reflections of its precious stones. (Icelandic, bifa, tremble, and rost, path.)
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The keeper of the bridge is Heimdall. It leads to Doomstead, the palace of the Norns or Fates.