#back-to-top {
    background-color: #169fe6;
    bottom: 0;
    box-shadow: 0 0 6px #169fe6;
    color: #444444;
    padding: 10px 10px;
    position: fixed;
    right: 50px;
    cursor: pointer;
    z-index:100;
}
#mainContent img
{
    z-index: 1000; //设置为最顶层，以免左侧导航栏遮挡
    position: relative;
    cursor: pointer;//鼠标样式：手势
    transition: all 1.2s; //放大的时间：1.2s
}
#mainContent img:hover {
    //transform: scale(1.4); //放大倍数：1.4倍
}