One of the most interesting artifacts I've had is something I was never supposed to see. The fellow from whom I got it said that it was smuggled out of a military research lab in the 1950s, and that I should keep it secret. Which I did for many years, until it was recently declassified. So now I feel free to show it here.
This device is called a blackout bulb, and its remarkable property is that when turned on, it emits only darkness.
You can imagine how important this device could be, useful for concealing virtually anything in darkness.
The first picture below shows it when it's turned off.
The second picture was taken by a high-speed camera in the split-second before it is fully deluminated.
The third picture is a photograph of the bulb when it is working.