I couldn't figure out how the prisoners were not able to see whether they were being watched or not. Reading through the article though, it is left as an open engineering solution.
I found the following bits valuable -
> By Blinds, and other contrivances, the Inspectors concealed from the observation of the Prisoners
Since this is also tied up with the surveillance society which we seem to be heading towards, I find the following point very important -
> Bentham's inspection principle applied not only to the inmates of the panopticon prison, but also the manager. The unaccountable gaoler was to be observed by the general public and public officials. The apparently constant surveillance of the prison inmates by the panopticon manager and the occasional observation of the manager by the general public