Uncaught TypeError: undefined is not a function

<div class="photo1 sliderPhoto">d</div>
<div class="photo2 sliderPhoto">d</div>
<div class="photo3 sliderPhoto">d</div>
<div class="photo4 sliderPhoto">d</div>

.sliderPhoto{
    position: absolute;
    top: 0px;
    left: 0px;
    background-position: center center;
    background-size: cover;
    width: 100%;
    height: 100%;
}
.photo1{
    background-image: url('../photos/ph1.jpg');
    z-index: 6;
}
.photo2{
    background-image: url('../photos/ph2.jpg');
    z-index: 6;
    display: none;
}
.photo3{
    background-image: url('../photos/ph3.jpg');
    z-index: 6;
    display: none;
}
.photo4{
    background-image: url('../photos/ph4.jpg');
    z-index: 6;
    display: none;
}

$photos = [$(".photo1")[0], $(".photo2")[0], $(".photo3")[0], $(".photo4")[0]];
$photos[0].css("z-index");