I'm trying to remember the name of a short story that's written in the form of a letter to a doctor/professor (I think). The writer of the letter/essay, who is a scientist, describes an experience with a time machine with himself and another man. They end up getting stranded somewhere and meeting some society. The society is suspicious of them, but I think they interact a little via gifts. Then somehow something terrible happens related to escalating tension with the society (I think the writer and his friends repeatedly do research or use items that are considered forbidden to the society), and the writer's friend ends up dying (I think there was a gun involved). Thanks for the help!