The Problem of the Green Capsule

Eye-witnesses are unreliable.
Marcus Chesney set up a test to demonstrate this where nothing was as it seemed.
But, alas, things were not as they seemed to Marcus Chesney either, and it cost him his life.
It takes Gideon Fell to unravel this web of delusion and illusion upon illusion.
Another classic John Dickson Carr puzzler.