Abstract
The relation between grain density in photographic tracks and particle energy has been derived theoretically assuming that the probability that a grain be developed is given by P=c[1exp[b(dEdR)12]. Using a single pair of parameters b and c we can represent with this equation all experimental curves so far published for alpha-particles, tritons, protons, π and μ mesons (University of Bristol). The agreement between experiments and theory is very satisfactory.