Historic bubble chamber photograph from the Argonne National Laboratory. When this picture is viewed, the large fiducial mark ("X"), should be at the top. The blurred circle at the bottom of the picture is caused by the reflection of light in the bubble chamber system. Nuclear particle tracks in the ten-inch bubble chamber mounted inside a superconducting magnet at Argonne show what happened to two negative K mesons that entered the bubble chamber from Argonne's Zero Gradient Synchrotron. The mesons entered the bubble chamber from the right. Most prominent are the tracks of bubbles resulting from the interaction of a negative K meson and a helium nucleus, which took place about two-thirds the way across the picture, slightly above its center. Five nuclear particles were emitted - two negative pi mesons, a deuteron, (heavy hydrogen nucleus), and two protons. Corkscrew circles in the picture are caused by stray electrons.