A Mimosa is a cocktail which consists of Fresh Orange Juice, and Champagne. It's also known as a Buck's Fizz.
Artistry of Mixing Drinks, by Frank Meier, 1936
Mimosa or Champagne Orange
"In a large wineglass: a piece of Ice, the juice of one-half Orange; fill with Champagne stir and serve."
Colin Peter Field's "The Cocktails of the Ritz Paris"
Mimosa (Frank Meier, 1925, apparently)
"I have read in a few books that we actually invented the Mimosa in 1925 at the RItz Paris. However, in Frank Meier's book of 1933 he cites the Mimosa or Champagne Orange and does not sign it himself. Can it be safe to assume that the Ritz Paris did not invent this cocktail? Did Frank forget to sign his creation?"
Three parts champagne and two parts thoroughly chilled orange juice