setInterval clearInterval

[inSide-j00ls]

I have the following function that starts downward scrolling:

function NachUnten (mausaufbild) {
var aktiv = window.setInterval("window.scrollBy(0, +10)", 50);}

this works nicely - but calling a second function with clearInterval(aktiv); does not work - only calling clear at the same time cancels the scrolling process but then I cant scroll at all - even if I call the same function with a variable that decides whether or not to call clearInterval it never works. How can I reference the interval once I have started it ?

Help much appreciated!