Adding lightning application in vf page

<aura:application access="GLOBAL" extends="ltng:outApp"> 

    <aura:dependency resource="ui:button"/>

</aura:application>

VF Page :

<apex:page>

    <apex:includeLightning />

    <div id="lightning" />

    <script>

        $Lightning.use("c:app15", function() {

        $Lightning.createComponent("ui:button",{ label : "Press Me!" },

                                   "lightning",function(cmp) {});

        });

    </script>

</apex:page>