Put the lemon juice and rind, hot water, salt, one-half cup of the sugar and the well-beaten egg yolks in the top part of a double boiler. Cook, stirring occasionally, for about fifteen minutes, until thickened.
Beat the egg whites until stiff; add the remaining one-quarter cup sugar gradually while beating.
Fold the hot lemon mixture into the beaten egg whites. Pour the mixture into the baked pie shell. Bake for about fifteen minutes, until delicately browned.