There used to be a lot of these camera obscura things around – you could walk into one and look around. It's like being inside a big camera – but instead, you're in a dark room – just what those two words mean in Latin. There's a small aperture on the roof – like the pinhole in a pinhole camera – and a series of mirrors or prisms that directs the beam of natural light from that aperture down, usually, onto a white, flat disc in the center of the dark room. There you see what is happening in the outside world. It's kind of neat – like being inside the single-lens reflex Nikon, in the dark, watching the images that strike the sensor. Cool.
There's an explanation and a history of such things here – from Aristotle through to Leonardo Da Vinci, who gave two clear descriptions of the camera obscura in his notebooks, to Giovanni Battista Della Porta, in his 1558 book Magiae Naturalis, recommending the use of this sort of device as an aid for drawing for artists. And they had military uses – from the defense of Venice in 1859 to military aviation applications in the mid-thirties, just before World War II.
But these optical curiosities became, mainly, tourist attractions at parks and at the seashore, a nineteenth century thing – and then they were gone. There used to be one in New York, in Central Park. It's gone now. But there still one in Santa Monica – on Ocean Boulevard, at the Senior Recreation Center in Palisades Park, overlooking the beach and the Santa Monica Pier below. And it's fairly new, installed in 1955, replacing one from the nineteenth century that was torn down long ago.
No one much visits – you have to ask for the key, as they keep it locked – and you give them your driver's license as collateral, so to speak. But ask for the key, and you can spend an hour alone inside a big camera. That's just what it is.
Everyone should do such things once in their life.