in drinking bumpers, means that the wine-glass is not full to the brim; between the wine and the rim of the wine-glass light may be seen. Toast-masters used to cry out, “Gentlemen, no daylights nor heeltaps”—the heeltap being a little wine left at the bottom of the glass. The glass must be filled to the brim, and every drop of it must be drunk.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Daylight from Infoplease: