if (document.images)
	{
	// main navigation
	home_on = new Image();
	home_on.src="images/nav/home_on.gif";
	about_on = new Image();
	about_on.src="images/nav/about_on.gif";
	portfolio_on = new Image();
	portfolio_on.src="images/nav/portfolio_on.gif";
	contact_on = new Image();
	contact_on.src="images/nav/contact_on.gif";
	}

function roll(whichone, turnon)
	{
	var root = "/images/";
	var x = (root.substring(0,1) == "/")  ? 1 : 0;
	var t = document.images[whichone].src;
	var subdirectory = t.substring(t.indexOf(root)+x, t.lastIndexOf('/')+1);
	var imageType = t.substring(t.lastIndexOf('.'));
	var direction = ( turnon ) ? "_on" : "_off";
	document.images[whichone].src = subdirectory + whichone + direction + imageType;
	}