/** Event handler for mouse wheel event.
         *鼠標滾動事件
         */ 
        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("鼠標滑輪向下滾動：" + delta + "次！"); // 1 
                $("btn_next_pic").onclick(random_num); 
                return; 
            } else { 
                // alert("鼠標滑輪向上滾動：" + delta + "次！"); // -1 
                $("btn_last_pic").onclick(random_num); 
                return; 
            } 
        }