js鼠标滑轮滚动事件绑定（兼容主流浏览器）

   /** 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;  
                }  
            }