In 1860, still not knowing of Stewart's measurements for selected qualities of radiation, Kirchhoff pointed out that it was long established experimentally that for total heat radiation, of unselected quality, emitted and absorbed by a body in equilibrium, the dimensioned total radiation ratio ⁠ E (T, i) / a (T, i) ⁠, has one and the same value common to all bodies, that is, for every value of the material index i. Again without measurements of radiative powers or other new experimental data, Kirchhoff then offered a fresh theoretical proof of his new principle of the universality of the value of the wavelength-specific ratio ⁠ E (λ, T, i) / a (λ, T, i) ⁠ at thermal equilibrium. His fresh theoretical proof was and still is considered by some writers to be invalid.