/** 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;

                }

            }