var activeDiv = '';

function make_visible(targetDiv)
{
    var previous;
    if (activeDiv != '')
    {
        previous = document.getElementById(activeDiv);
        if (activeDiv != targetDiv)
        {
            previous.className = "Invisible";
        }
    }
    var newActive = document.getElementById(targetDiv);
    newActive.className = "Visible";
    activeDiv = targetDiv;
}