Carl D. Anderson, Positive Electron, (1933)
The above image is a cloud chamber photograph of the first positive electron (positron) ever observed.
The original caption on the photograph reads as follows:
A 63 million volt positron (Hρ = 2.1×105 gauss-cm) passing through a 6 mm lead plate and emerging as a 23 million volt positron (Hρ = 7.5×104 gauss-cm). The length of this latter path is at least ten times greater than the possible length of a proton path of this curvature.
First published in the Physical Review in March 1933.