In a small saucepan, combine the lemon juice and sugar over medium heat. Let cook, stirring occasionally, until the sugar is completely dissolved. Transfer to a bowl, pitcher, or large liquid measuring cup, cover, and chill until ready to serve.

While the syrup is chilling, puree the raspberries in a food processor or food mill, or mash them gently with the back of a spoon. If desired, press the berry puree through a fine mesh strainer. (This is not necessary, but does remove the seeds, if you'd rather not have those in your drink.) 

To serve, spoon a small amount of the berry puree into each serving glass. Add ice if desired. Pour 2 cups of the sparkling water into the chilled lemon syrup. Taste and add more sparkling water to taste. Pour the lemonade into the glasses and serve immediately.