Boil milk, let it cool till luke-warm, take 3 eggs beat well, add the milk, put into jug.  Stand in a saucepan of cold water till it gradually comes to the boil, stirring it occasionally.  Do not let the custard boil.  Sweeten to taste, essence of lemon if preferred.

From

The Housewife's cookery book and shopping guide. 5th edition.