问google.maps.geometry.spherical错误

EN

var x=new google.maps.LatLng(52.395715,4.888916);
var stavanger=new google.maps.LatLng(58.983991,5.734863);
var amsterdam=new google.maps.LatLng(52.395715,4.888916);
var london=new google.maps.LatLng(51.508742,-0.120850);
function initialize()
{
var mapProp = {
  center:x,
  zoom:4,
  mapTypeId:google.maps.MapTypeId.ROADMAP
};

var map=new google.maps.Map(document.getElementById("googleMap"),mapProp);
var myTrip=[stavanger,amsterdam,london];
var flightPath=new google.maps.Polyline({
    path:myTrip,
    strokeColor:"#0000FF",
    strokeOpacity:0.8,
    strokeWeight:2
    });
flightPath.setMap(map);

try{
    var markerpos = google.maps.geometry.spherical.interpolate(flightPath.getAt(0), flightPath.getAt(1), .5);
    // also tried //
    //var Spherical = google.maps.geometry.spherical;
    //var markerpos = Spherical.interpolate(flightPath.getAt(0), flightPath.getAt(1), .5);
    //var markerpos = google.maps.geometry.spherical.interpolate(amsterdam, london, .5);
}
catch(ex){alert(ex);}

var marker = new google.maps.Marker({position: markerpos,
    map: map,  clickable: false
});
}