/** Event handler for mouse wheel event.          *鼠标滚动事件          */          var wheel = function(event) {              var delta = 0;              if (!event) /* For IE. */                  event = window.event;              if (event.wheelDelta) { /* IE/Opera. */                  delta = event.wheelDelta / 120;              } else if (event.detail) {                  /** Mozilla case. */                  /** In Mozilla, sign of delta is different than in IE.                  * Also, delta is multiple of 3.                  */                  delta = -event.detail / 3;              }              /** If delta is nonzero, handle it.              * Basically, delta is now positive if wheel was scrolled up,              * and negative, if wheel was scrolled down.              */              if (delta)                  handle(delta);              /** Prevent default actions caused by mouse wheel.              * That might be ugly, but we handle scrolls somehow              * anyway, so don't bother here..              */              if (event.preventDefault)                  event.preventDefault();              event.returnValue = false;          }          /** Initialization code.           * If you use your own event management code, change it as required.          */          if (window.addEventListener) {              /** DOMMouseScroll is for mozilla. */              window.addEventListener('DOMMouseScroll', wheel, false);          }          /** IE/Opera. */          window.onmousewheel = document.onmousewheel = wheel;          /** This is high-level function.          * It must react to delta being more/less than zero.          */          var handle = function(delta) {              var random_num = Math.floor((Math.random() * 100) + 50);              if (delta < 0) {                  // alert("鼠标滑轮向下滚动：" + delta + "次！"); // 1                  $("btn_next_pic").onclick(random_num);                  return;              } else {                  // alert("鼠标滑轮向上滚动：" + delta + "次！"); // -1                  $("btn_last_pic").onclick(random_num);                  return;              }          }