The mental models by early actors are presented. Sect 4.1 discusses Isaac Newton’s “globuli of light” along with amendments by some of his important adherents; the next two sections cover Einstein’s mental model of light quanta around 1909 as singularities in the radiation field, along with his doubts about it 1910–15. Sects. 4.4–4.6 review the mental models of three influential experimental physicists: Johannes Stark’s light quanta, J.J. Thomson’s model of hard x rays, and W.H. Bragg’s neutral pair model for -rays. Sect. 4.7 treats the mental model argued by Planck, Peter Debye and Arnold Sommerfeld to relegate the quantization of energy and momentum to the material resonators in a black body. Sect. 4.8 deals with Gilbert Lewis’s mental model of photons by which this American physical chemist introduced the term ‘photon’ in his reflections about the temporal symmetry between emission and absorption but still attaching to it the completely wrong notion of photon number being conserved.