I'm currently writing three novels about Father Junipero Serra's efforts to establish missions in the Californias during the late 18th Century.
I came across a point where I needed the address of the Viceroy of New Spain - and I can't seem to find out how it was done.
An archbishop is "Excellence" unless he's a Cardinal, then it's Eminence.
So, how was a Viceroy addressed? English AND/OR Spanish is just fine.