As soon as I saw the latest xkcd comic by Randall Munroe, I had to look up "spinthariscope" to see if it was a real thing. And it is. William Crookes invented it by accident during his nuclear experiments in 1903. He spilled a tiny amount of radium bromide (a radioactive salt) onto a thin screen of zinc sulfide. Since radium bromide was a very expensive material, he carefully picked up every speck, using a magnifying glass to see them. He noticed flashes of light, produced by the radium bromide throwing off alpha particles. This was a pretty neat discovery, so Crookes fashioned an enclosed device for observing the effect. That's how the spinthariscope was born.
As a scientific instrument, the spinthariscope soon became obsolete, but it was still impressive to non-physicists and kids. In 1947, you could order one from the back of a cereal box. In the 1950s, a small spinthariscope was included in the Chemcraft Atomic Energy Lab for children.
(Image credit: Theodore Gray)
What's more, United Nuclear still sells spinthariscopes, although their modern version uses thorium instead of radium bromide.