[jQuery] How to Get Div from IFrame

[Danish]

I have a page with multiple Frames and Division, Now I am trying to
extract one div having id="my_div" from iframe with Id =
"canvas_frame" but I am fail to do so, Nor I am getting iframe neither
Division in iframe.
I tried all these selectors but always null returns although they are
in page:
1) $("my_div");
2) $("my_div","canvas_frame");
3) $("canvas_frame",content.document);
4) $("canvas_frame");
5)  var $frame = $("canvas_frame",content.document);
     var $div = $("my_div",$frame);

I am able to do this with Pure JavaScript but due to some reasons I
want to use JQuery Library, Any help will be highly appreciated.