This cloud chamber photograph (the right half is a mirror image) shows the tracks of a shower of electrons produced by a cosmic ray. Robert A. Millikan gave the name “cosmic rays” to the charged particles (electrons and atomic nuclei) that bombard the earth from all directions in space, and he devoted much of his later research to characterizing this radiation. From this pioneering work, continued by his students, cosmic rays became the key to the discovery of many of the fundamental particles of matter.
