var first_opac = 20;
var sec_opac =100;
var opac = first_opac;
var speed = 6;
var si;
var cleared = true;

function showimage(what,doshow) 
{
     if (document.all) 
     {
          cleared = false;
          (doshow) ? opac+=speed : opac-=speed;
          eval(what+".filters.alpha.opacity="+opac);
          
          if (opac <= first_opac || opac >= sec_opac) 
          {
               clearInterval(si);
               cleared = true;
          }
     }

     if (document.layers) clearInterval(si);
}

function ClearInt() 
{
     if (!cleared) clearInterval(si);
}