 new THREE.Mesh( planeGeo, matTop );
                planeTop.name = "top";
                planeTop.position.y = 10;
                planeTop.rotateX( Math.PI / 2 );
                walls.add(planeTop);

                var matBack = new THREE.MeshPhongMaterial( { color: 0x808080, side:THREE.DoubleSide, opacity:0.3, transparent:true });
                var planeBack = new THREE.Mesh( planeGeo, matBack );
                planeBack.name = "back";
                planeBack.position.z = -10;
        