MR. LANDSBERG‘S letter, in that it derives a general transformation of which the Lorentz transformation is a special case, is doubtless of importance mathematically. It seems to me to have considerable physical interest for two reasons. First, it should help to elucidate the difference between the ‘one-‘way character of the laws of radiation and the ‘two-‘way character of the laws of motion, since it shows that both sets of laws, expressed in relativistic form, are special cases of a more general relation. Secondly, the detailed correspondence obtained between the Lorentz transformation and the transformation operative in thermal relativity should help by analogy in the further development of the latter. It might help, for example, in deriving a rigorous relation between specific heat and temperature (expressed, of course, not in those terms but in terms of the thermal relativity measurements) by analogy with the mechanical relation between mass and velocity.