docs: function(opacity) { 
          if (opacity != undefined) {
              this.opacity = opacity; 
          }
          
          if (this.div != null) {
              // for Mozilla and Safari
              this.div.style.opacity = this.opacity;
  
              // for IE
              this.div.style.filter = 'alpha(opacity=' + this.opacity*100 + ')';
          }
      }