This is one of several stories which appeared in the This Week supplement to a number of North American newspapers, but does not seem to have been published in Britain.
Old Man Thornton is a lighthouse-keeper on the west African coast. A man and a woman in a sailing dinghy land on his rock during a storm. They accidentally show him that they are carrying a large sum of cash. Once he has seen it, he will have to die in case he reports it, but meanwhile he has to keep the light burning until the storm is over. He is watched to ensure he does not send any signals using the light. Will he be able to send one none the less?
Published in This Week Magazine, 7 October 1956.
Included in comic strip form as "Message to the mainland" in "The three riddles", Super Detective Library No 106, 1957.