How to allow http content within an iframe on a https site

I load some HTML into an iframe but when a file referenced is using http, not https, I get the following error:

[blocked] The page at {current_pagename} ran insecure content from {referenced_filename}

Is there any way to turn this off or any way to get around it?

The iframe has no src attribute and the contents are set using:

frame.open();
frame.write(html);
frame.close();