The Problem of the Green Capsule, aka The Black Spectacles

In the small English village of Sodbury Cross, pretty Marjorie Wills is suspected of having poisoned some chocolates in the local tobacco-and-sweet shop, using a method pioneered by historical poisoner Christiana Edmunds.
Her uncle, wealthy Marcus Chesney, believes that eyewitnesses are unreliable. He avers that to observe something, then to relate accurately what was just seen, is impossible. In order to prove his statements, he sets up a test; three witnesses are invited to witness some staged events not only in their view but in that of a movie camera.
After the events, it is planned that they will answer a list of ten questions. Marcus Chesney takes a principal role in the staged events and, during them, is fed a large green capsule containing poison by a masked and disguised figure wearing black spectacles. Amazingly, the three witnesses cannot agree upon the answers to any of the questions and no one can identify the murderer.