Methods for the experimental determination of the strain-optical constants pij of diamond are described and compared, starting with the pioneering work published in 1947 by G. N. Ramachandran (1921–2001). The review covers the period before the 1970s when Brillouin scattering techniques were developed for directly measuring both the elastic stiffness constants cij and the pij. Before then, various means for mechanical stressing of the specimen were necessarily employed in pij measurement. These yielded the piezo-optical (stress-optical) constants qij, from which derivation of the pij depended upon the accuracy with which the elastic constants cij were known. The first mechanical experiments applied only uniaxial stresses; precision was limited by the maximum stress usable and the shapes and perfection of the specimens available. Later techniques, developed in the 1960s, which used hydrostatic compression, directly found the physically significant ratio ρ(dn/dρ) to be negative (ρ = density, n = refractive index) and thus provided a direct answer to the interesting question ‘Does the refractive index of diamond (and its dielectric constant) increase or decrease as the crystal is hydrostatically compressed?’ The early Bristol hydrostatic compression work of Gibbs and Hill [D.F. Gibbs and G.J. Hill, Phila. Mag. 9 (1964) 367.] is re-examined and is defended against an implication of error concerning this question. Currently there is satisfactory agreement between experiment and computed pij of diamond.