Grate the lemon rinds finely, avoiding all pith. Squeeze out the juice and strain it. Melt the butter in a double saucepan or bowl over hot water, and add the lemon rind, juice and sugar. When the sugar has dissolved, stir in the well-beaten eggs. Cook gently, stirring well until the mixture thickens. Pour into small hot jars and cover, or else cool and pour into small freezer containers.