if (document.images) {

plaatje1on = new Image();
plaatje1on.src = "http://www.fibonicci.com/images/buttons/button-easy-green.png";
 
plaatje2on = new Image();
plaatje2on.src = "http://www.fibonicci.com/images/buttons/button-medium-yellow.png";
 
plaatje3on = new Image();
plaatje3on.src = "http://www.fibonicci.com/images/buttons/button-hard-red.png";
 
plaatje4on = new Image();
plaatje4on.src = "http://www.fibonicci.com/images/buttons/button-answers-blue.png";
 
plaatje5on = new Image();
plaatje5on.src = "http://www.fibonicci.com/images/buttons/button-retry-blue.png";

plaatje6on = new Image();
plaatje6on.src = "http://www.fibonicci.com/images/buttons/button-back-blue.png";

plaatje7on = new Image();
plaatje7on.src = "http://www.fibonicci.com/images/buttons/button-next-blue.png";

plaatje1off = new Image();
plaatje1off.src = "http://www.fibonicci.com/images/buttons/button-easy-gray.png";
 
plaatje2off = new Image();
plaatje2off.src = "http://www.fibonicci.com/images/buttons/button-medium-gray.png";
 
plaatje3off = new Image();
plaatje3off.src = "http://www.fibonicci.com/images/buttons/button-hard-gray.png";

plaatje4off = new Image();
plaatje4off.src = "http://www.fibonicci.com/images/buttons/button-answers-gray.png";

plaatje5off = new Image();
plaatje5off.src = "http://www.fibonicci.com/images/buttons/button-retry-gray.png";

plaatje6off = new Image();
plaatje6off.src = "http://www.fibonicci.com/images/buttons/button-back-gray.png";
 
plaatje7off = new Image();
plaatje7off.src = "http://www.fibonicci.com/images/buttons/button-next-gray.png";
}

 
function muisop(imgname) {
if (document.images) {
document[imgname].src = eval(imgname + 'on.src');
}
}
 
function muisaf(imgname) {
if (document.images) {
document[imgname].src = eval(imgname + 'off.src');
}
}

function CheckKeyCode()
{
if( (event.keyCode == 13 || event.keyCode == 16) ||
(event.keyCode >= 58 && event.keyCode <= 95) || 
(event.keyCode >= 106 && event.keyCode <= 108) ||
(event.keyCode >= 111 && event.keyCode <= 187) ||
(event.keyCode >= 191 && event.keyCode <= 222) ) {
return false; }
else {
return true;
}
}