The mid-1930s to 1950s period established a paradigm centered on direct particle identification within cosmic rays and on the fundamental interactions of high-energy charged particles with matter. Researchers pursued unambiguous signatures of new particle species, measured energy loss and penetration in detectors, and began to map long-term flux variations influenced by solar activity and galactic origins. Theoretical and experimental work converged around characterizing how cosmic rays originate, propagate, and appear as a mosaic of secondaries shaped by magnetic fields, density, and polarization of media.