In a small mixer bowl beat the yolks with 1/2 c of sugar at high speed for 5 minutes, until the mixture is very pale and thick. On low speed, gradually add the lime juice. Stir in the grated rind. Turn the mixture into the top of a large double boiler over shallow hot water on moderate heat. Cook for 6 or 7 minutes, stirring constantly and scraping sides and bottom. Mixture should reach 175 or 180 degrees, and thicken enough to coat a wooden spoon. Remove the top of the double boiler and pour mixture into a large mixing bowl. Stir occasionally until it cools. In a large mixer bowl, add the salt to the egg whites and beat until soft peaks form. Reduce speed and gradually add the remaining 1/4 c sugar. Increase speed again and beat until the mixture holds a definite shape, but not until stiff or dry. Gradually, in three additions, fold the yolk mixture into the whites, handling very little and very carefully. It is not necessary to fold the first two additions thoroughly. Turn the mixture into the crust. It will look too full, but just mound it high. Bake for 15 minutes until the top is lightly browned. Cool on a rack to room temperature, then freeze. When frozen, cover with plastic wrap. The pie may be kept frozen for up to 3 weeks. Serve frozen.