I'll play. I voted that he saw the Father.
Why? Because the context is speaking of the Father, sitting on the throne. The Lamb that was slain was right there in the "midst" of the throne. We do know that Jesus sits at the right hand of the Father.
I don't believe this idea that God is pure and incorporeal spirit. The Father sits, has hands, speaks, etc...Yes, God is Spirit. He is also Love. He is also Truth. He also has a face, head, hands, feet, etc.