function init() {
		var windowScreenWidth = 0, windowScreenHeight = 0;
	  	if( typeof( window.innerWidth ) == 'number' ) {
	    	//Non-IE
	    	windowScreenWidth = window.innerWidth;
	    	windowScreenHeight = window.innerHeight;
	  	} else if( document.documentElement && ( document.documentElement.clientWidth || document.documentElement.clientHeight ) ) {
	    	//IE 6+ in 'standards compliant mode'
	    	windowScreenWidth = document.documentElement.clientWidth;
	    	windowScreenHeight = document.documentElement.clientHeight;
	  	} else if( document.body && ( document.body.clientWidth || document.body.clientHeight ) ) {
	    	//IE 4 compatible
	    	windowScreenWidth = document.body.clientWidth;
	    	windowScreenHeight = document.body.clientHeight;
	  	}
	  
		var windowLeft = windowScreenWidth/6;
		var windowTop = windowScreenHeight/5;
		var windowWidth = windowLeft * 4;
		var windowHeight = windowTop * 3;
		var content = document.getElementById("div_content");
			
		content.style.left = windowLeft + "px";
		content.style.top = windowTop + "px";
		content.style.width = windowWidth + "px";
		content.style.height = windowHeight + "px";
		content.style.visibility = 'visible';
	}
	
	function submitformgastenboek() {
		if (document.gastenboekform.gastenboektitel.value == '') {
			alert('Een titel is verplicht');
			return false;
		} else if (document.gastenboekform.gastenboektext.value == '') {
			alert('Een bericht is verplicht');
			return false;			
		} else if (document.gastenboekform.gastenboeknaam.value == '') {
			alert('Uw naam is verplicht');		
			return false;			
		} else {
			document.gastenboekform.method = 'post';
			document.gastenboekform.submit();
		}
	}