JSONの取得について

javascriptでJSONのデータを取得しようとしています。
しかし、一部のデータ取得ができず、「Uncaught TypeError: Cannot read property 'url' of undefined」と表示されてしまいます。
自分では何度見ても間違えているように見えないので、どなたかどこが違うか教えて下さい。
なお、「Uncaught TypeError: Cannot read property 'url' of undefined」は
 var img = json[i].object.attachments.image.url;
の部分で出ています。

javascript

$(function(){
 $.ajax({
 type: "GET",
 url: "https://www.googleapis.com/plus/v1/people/***************/activities/public?key=***************************",
 dataType: "json",
 success: function(json){
 json = json.items;
 var num = json.length;
 var Feed = "<ul>\n";
 for(var i=0; i<num; i++){
 var title = json[i].title;
 var link = json[i].url;
 var icon = json[i].actor.image.url;
 var name = json[i].actor.displayName;
 var coment = json[i].title;
 var img = json[i].object.attachments.image.url;

  Feed +=  '<li><span><a href="' + link + '"><img src="' +icon+ '"></a>' + name + '</span>' +'<br>' + coment + '<p><img src="' +img+ '"></p>' + '</li>\n';
 }

 Feed += "</ul>\n";
 $("#topics").append(Feed);
 }
 });
});

上記URLで取得できたJSON

{
 "kind": "plus#activityFeed",
 "etag": "\"4OZ_Kt6ujOh1jaML_U6RM6APqoE/PrhYcqBsDYTg2BF_P19O8HwfpCw\"",
 "title": "Google+ List of Activities for Collection PUBLIC",
 "updated": "2016-04-15T07:16:23.244Z",
 "items": [
  {
   "kind": "plus#activity",
   "etag": "\"4OZ_Kt6ujOh1jaML_U6RM6APqoE/Z6VW6M1zVwCl3efyr8MmNgKP5KE\"",
   "title": "神秘的なシャボン玉♪",
   "published": "2016-04-15T07:16:23.244Z",
   "updated": "2016-04-15T07:16:23.244Z",
   "id": "z12jvreytoe5vnklo220eporttighvqd0",
   "url": "https://plus.google.com/114624284244149933026/posts/9nW4sCZCJai",
   "actor": {
    "id": "114624284244149933026",
    "displayName": "****",
    "url": "https://plus.google.com/114624284244149933026",
    "image": {
     "url": "https://lh5.googleusercontent.com/-Vy7vaAD81i0/AAAAAAAAAAI/AAAAAAAAABE/mnXdlb-q2VM/photo.jpg?sz=50"
    },
    "verification": {
     "adHocVerified": "UNKNOWN_VERIFICATION_STATUS"
    }
   },
   "verb": "post",
   "object": {
    "objectType": "note",
    "actor": {
     "verification": {
      "adHocVerified": "UNKNOWN_VERIFICATION_STATUS"
     }
    },
    "content": "神秘的なシャボン玉♪\ufeff",
    "url": "https://plus.google.com/114624284244149933026/posts/9nW4sCZCJai",
    "replies": {
     "totalItems": 0,
     "selfLink": "https://www.googleapis.com/plus/v1/activities/z12jvreytoe5vnklo220eporttighvqd0/comments"
    },
    "plusoners": {
     "totalItems": 0,
     "selfLink": "https://www.googleapis.com/plus/v1/activities/z12jvreytoe5vnklo220eporttighvqd0/people/plusoners"
    },
    "resharers": {
     "totalItems": 0,
     "selfLink": "https://www.googleapis.com/plus/v1/activities/z12jvreytoe5vnklo220eporttighvqd0/people/resharers"
    },
    "attachments": [
     {
      "objectType": "photo",
      "displayName": "神秘的なシャボン玉♪",
      "id": "114624284244149933026.6273678319375980162",
      "content": "shabon.jpg",
      "url": "https://plus.google.com/photos/114624284244149933026/albums/6273678322613280529/6273678319375980162",
      "image": {
       "url": "https://lh3.googleusercontent.com/-IgS76Ke2OGo/VxCVMS15loI/AAAAAAAAAD0/0JgRR4_bQiMM-bl3kIdce4Xu0a6F5H9Qg/w506-h750/shabon.jpg",
       "type": "image/jpeg"
      },
      "fullImage": {
       "url": "https://lh3.googleusercontent.com/-IgS76Ke2OGo/VxCVMS15loI/AAAAAAAAAD0/0JgRR4_bQiMM-bl3kIdce4Xu0a6F5H9Qg/w960-h639/shabon.jpg",
       "type": "image/jpeg",
       "height": 639,
       "width": 960
      }
     }
    ]
   },
   "provider": {
    "title": "Google+"
   },
   "access": {
    "kind": "plus#acl",
    "description": "Public",
    "items": [
     {
      "type": "public"
     }
    ]
   }
  },

上記JSON内の「https://lh3.googleusercontent.com/-IgS76Ke2OGo/VxCVMS15loI/AAAAAAAAAD0/0JgRR4_bQiMM-bl3kIdce4Xu0a6F5H9Qg/w506-h750/shabon.jpg」を取得しようとしています。

よろしくお願いします。