Daly's Bartenders' Encyclopedia by Tim Daly (1903, p.68)

3 dashesAbsinthe base
brandy — Float a small quantity on top.base
0.75 glassseltzer — syphon seltzerbase
  1. Fill a star champagne glass three-quarters full of syphon seltzer and add the absinthe.
  2. Float a small quantity of brandy on top and serve.

This is a useful tiny morning restorative checkpoint at the edge of absinthe service and cocktail culture.

The floated brandy top is the key structural move here.

See also: Soda Cocktail, Swiss Ess