Using JQuery to Access a New Window's DOM

In this case you are trying to access a dom wich has no jQuery enhancement. You need to first load a jquery.js into this document. If done the syntax would be.

var popup ='/some/url.html');

But be very careful, in multi document management and communication are many bugs and inconveniences between the many different browser versions and vendors.

It would really be better if you keep communication to a absolute minimum, and just load another complete html file into the popup.