Purée the raw raspberries in the liquidiser or through the fine blade of a mouli-légumes, and strain the purée through a fine sieve or strainer. Add the sugar and the juice of half a lemon. Beat well with a wire whisk to dissolve the sugar. Transfer the mixture to the ice-cream maker, and freeze.
To serve the sorbet, scoop it into balls with a spoon dipped into hot water and pile it into well-chilled ice-cream dishes.