Put two ounces of butter in a saucepan, adding two tablespoonfuls of flour while stirring ; moisten with one quart of water, and season with one tablespoonful of salt and half a tea-spoonful of pepper. Let it simmer on the side of the stove for thirty minutes until it thickens ; then add, little by little, half an ounce of butter, beating it continuously until it becomes perfectly white. Squeeze in the juice of a lemon ; stir once more, strain through a hair sieve and serve.