Blend the mustard and the oil, adding the latter, drop by drop at first, and then proceeding more confidently. Whenever the Dressing appears to be curdling, add a few drops of vinegar, and work rapidly till it becomes smooth again. Add the salt and pepper, and when the Dressing is finished use it for Celery Salad, fish, tomatoes, potatoes, etc. If the oil appears to separate from the other ingredients, it can always be rubbed into them smoothly again by adding a few drops of vinegar. In all these Salads the question of oil and its measurements can only be approximated. Good judgment must always be the final test.