Rub ½ lb sugar over the zest of ½ lemon, pound the sugar, and beat with 5 egg yolks until white. Dilute with 1½ glasses light white wine, add the juice from ½ lemon, and place the mixture on top of the stove. Beat with a whisk until it thickens, remove from the fire, and set on ice, constantly beating with the whisk. Add 1 glass isinglass and, if desired, ½ wineglass white rum. Whip again, then pile into a mold, and set on ice.