Under absolute radiation one understands the energy quantity, which traverses (in unit time) the unit surface of an absolutely resting plane situated perpendicular of the absolute beam direction. This energy quantity is equal to the heat, absorbed by the unit surface of an equally located black plane.
Under total relative radiation we understand the energy quantity, which traverses (in unit time) the unit surface of an (imagined) plane, moving with absolute velocity , and which is oriented perpendicular to the relative beam direction. If this imagined plane is replaced by a material black plane (moving and oriented in the same way), then this total relative radiation is not identical with the amount of the heat absorbed by the latter; because the work of the radiation pressure or of the (external) work against it, has to be added here additionally, depending on whether the black plane is moving in the negative or positive sense. If one decreases or increases the total relative radiation by the amount of this work, then one obtains the amount of heat absorbed by the black plane, which amount we want to call the true relative radiation.
This mode of conception, according to which mechanical work is directly transformed into radiation energy and vice versa, was first spoken out (as I believe) by Poynting[1] and then by v. Türin,[2] independent from any specific idea concerning the essence of radiating heat, and purely as the consequence of the energy theorem. This is also in agreement with the theory of Abraham. What we call here true relative radiation, simply corresponds to the "relative radiation" of Abraham.
Quite analogous are the relations at the emission of a moving black surface. It was left by a certain amount of total relative radiation, which is equal to the true relative radiation and is increased or decreased by the amount of work performed by or against the radiation pressure, depending on whether
