Gradually the yolk of egg and oil will begin to get thick, first of all like custard.

When this is the case a little more oil may be added at a time, but never more than a teaspoonful.

As more oil is added, and the beating continues, the sauce gets thicker and thicker, till it is nearly as thick as butter in summer time.

When it arrives at this stage no more oil should be added.

A little tarragon vinegar may be added at the finish, or a little lemon juice.