There is a camera obscura in Edinburgh, up near the castle, and there’s one in Bristol, near the Clifton Suspension Bridge. There are lots more, in various places, but those are the two I’ve visited. I’ve been asked to explain how the camera obscura works, and I hope to do so without having to use ‘Camera Obscura’ in the plural.
The Camera Obscura was invented, or discovered, probably by accident, long before lenses and telescopes or indeed cameras in the modern English sense. Somehow I suspect the ancient Egyptians would have been the first to make one deliberately. ‘Camera Obscura’ translates simply as ‘Dark Room’, and someone noticed that if there’s just a chink in the wall of a really dark room, you can see, projected onto the wall opposite the chink, an inverted image of the world outside. (Assuming of course it’s light out there.)(Duh.)
Having seen some of Michael Faraday’s diagrams I no longer feel so ashamed of my own. The diagram below may or may not clarify matters. I have added a diagram of the pinhole camera, which works the same way. You can actually take a real photograph with a pinhole camera, just by putting a piece of photographic film where the tracing-paper would go.
In a modern (that is to say, post-Galilean) camera obscura, the chink in the wall is replaced by a lens up on the roof, facing out horizontally and movable by a long handle from the room itself. There’s a prism or mirror behind the lens, so the image is projected down to a horizontal, preferably slightly dished, screen below, round which spectators can gawp without getting in the way of the beam.