I'm looking for "The Staging of Eighteenth-Century Designs for Scenery" by Richard Southern (in Journal of the Royal Institute of British Architects,
Unfortunately, we do not have the Journal of the Royal Institute of British Architects available in any format. I did check and it appears UT@Austin does have the journal title available for the time period you indicated in your question. We do have several books by the author in our collection, but not the journal title.
Please let us know if you have any other questions.