function next_screen() 
{  
    var scr1 = document.getElementById("screen1");     
    var scr2 = document.getElementById("screen2");     
    scr1.style.display = 'none';     
    scr2.style.display = 'inline';   
}  

function prev_screen() 
{     
    var scr1 = document.getElementById("screen1");     
    var scr2 = document.getElementById("screen2");     
    scr2.style.display = 'none';     
    scr1.style.display = 'inline';  
}