javascript - css("margin-left")位null??? 迷茫了

<p  id="part4_slide">
            <p><img id="p4_img1" src="img/air/air_part4_1.jpg"  alt="pictrue" ></p>
            <p><img id="p4_img2" src="img/air/air_part4_1.jpg"  alt="pictrue" ></p>
                         
        </p>

下面是css:

#p4_img1{
    width:865px;
    height:130px;
    
    }
#p4_img2{
    width:865px;
    height:130px;
    margin-left:-180px;
    }

下面是js:

var arr_img=[$("p4_img1"),$("p4_img2"),$("p4_img3"),$("p4_img4"),$("p4_img5")];

var left=arr_img[0].css("margin-left");

alert(left);//null??????
alert(arr_img[0].width())//null???????