1/2 pint of oil, the yolk of 1 egg, the juice of a lemon, 1/2 teaspoonful each of mustard, pepper, and salt. Place the yolks in a basin, which should be quite cold; work them smooth with a wooden spoon, add the salt, pepper, and mustard, and mix all well. Stir in the oil very gradually, drop by drop; when the sauce begins to thicken stir in a little of the lemon juice, continue with the oil, and so on alternately until the sauce is finished. Be sure to make it in a cool place, also to stir one way only. It you follow directions the sauce may curdle; should this ever happen, do not waste the curdled sauce, but start afresh with a fresh yolk of egg, stirring in a little fresh oil first, and then adding the curdled mixture.