1. Quantum Theory of Radiation
2. Such an interpretation seems to be suggested by W. Heisenberg in his old but excellent book I principi fisici della teoria dei quanti (Torino, 1953).
3. Obviously all that is said for a photon can be repeated, for instance, for any other particle, including the neutrino, for which the present author, at the appropriate time, tried to establish through a model similar to the one proposed by Fermi that it is possible to speak of a «propagation» with a velocity equal to that of light (B. Ferretti: Nuovo Cimento, 14, 70 (1937)). In the case of fermions, since there is no classical field, local properties can only be given for the currents, and it is known that also for the currents the number of particles must remain indefinite.
4. Rigorously, within this context, a photon cannot be considered as a physical system (like a hydrogen atom in nonrelativistic quantum mechanics) but is should be considered as a «particular state» of the physical system of electromagnetic radiation.
5. Presented by the author at the meeting of the Accademia delle Scienze dell'Istituto di Bologna of Febraury 26, 1967.