That scratch going up the left half of the picture might look like an accidental blemish, but it’s actually the first trace of a positron (a fundamental unit of antimatter) ever recorded. That thick line across the middle is a lead barrier. The thing went through it from the bottom to the top, we know, because the top part is more curved. The line could have been left by a proton, but it’s 10 times too long.
So this is an electron with a positive charge. Nowadays, they’re commonly used in every hospital – the “P” in PET scan stands for “positronic.” We use antimatter to look inside people. In 1932, though, this little scratch was kind of amazing.