The Imperial Fizz (also called a Whiskey Fizz). A fizz style cocktail is one which uses water and sugar for the fizz, its "Imperial" title suggests a drink of superior quality and would traditionally have been made with a blended whiskey (if only because until the 1960s, blended whiskey is all anyone drank).
In a cocktail shaker mix the lemon juice and caster sugar until the sugar has dissolved.
Add the whisky and half-fill with ice.
Shake until well-chilled.
Strain into a highball glass over fresh ice and top with soda water.