"joined me... and I assigned him to the task of continuing the curvature measurements... As more data accumulated... practically all of the low-velocity cases of positive charge were particles... whose mass seemed to be too small to permit their interpretation as s. The alternative explanations... were that these particles were either ordinary electrons (of negative charge) moving upward, or some unknown lightweight particles of positive charge moving downward. In the spirit of scentific conservatism I tended... toward the former... [[Robert Andrews Millikan|[T]he chief]]... repeatedly pointed out that cosmic ray particles travel downward, and not upward, except in extremely rare circumstances, and that these... must be downward-moving protons. This point of view was difficult for me to accept... since in nearly all cases the density of the... tracks... was too low for particles of proton mass. To resolve this apparent paradox, a plate was inserted across the center of the ... [A] fine example was obtained in which a low-energy lightweight particle of positive charge was observed to traverse the plate... This particle came in from the bottom of the chamber, passed through the lead plate and went out near the top of the chamber. ...[I]ts track... was more curved above the plate... this meant it was going slower... therefore, it must have passed through the plate traveling upward."