Preheat oven to 350 degrees F (175 degrees C). Grease and flour a 10 inch tube pan. Sift together the flour and baking powder. Set aside.
In a large bowl, cream together the butter and sugar until light and fluffy. Beat in the yolks one at a time, then stir in the lemon juice and vanilla. Beat in the flour mixture alternately with the milk.
In a large glass or metal mixing bowl, beat egg whites until stiff peaks form. Fold 1/3 of the whites into the batter, then quickly fold in remaining whites until no streaks remain. Pour batter into prepared pan.
Bake in the preheated oven for 75 minutes, or until a toothpick inserted into the center of the cake comes out clean. Let cool in pan for 10 minutes, then turn out onto a wire rack and cool completely.
