Set the thermostat of the refrigerator to its coldest setting.
In a saucepan, combine the black- currant juice, water, sugar and lemon rind.
Bring to the boil over moderately high heat and boil for 4 minutes.
Remove the pan from the heat and stir in the lemon juice and dissolved gelatine.
Pour the mixture through a strainer into a freezer tray.
Place the freezer tray in the frozen food storage compartment of the refrigerator and chill for 30 minutes.
In a small bowl, beat the egg white with a wire whisk or rotary beater until it forms stiff peaks.
Whisk the blackcurrant mixture into the egg white.
Spoon the mixture back into the freezer tray and return to the storage compartment.
Freeze the mixture for a further 1 hour.
Remove the freezer tray from the refrigerator and turn the blackcurrant mixture into the mixing bowl.
Whisk for 1 minute.
Return the mixture to the freezer tray and replace it in the storage compartment.
Continue whisking every hour for 4 hours, then leave the iced mousse to freeze overnight.
With a sharp knife, slice the melon crossways into six slices.
Scoop out the seeds and place the slices on individual plates.
Spoon the mousse into the centre of each piece.