While in the preceding part the phenomena of radiation have been presented with the assumption of only well known elementary laws of optics summarized in [sect:2.] Sec. 2, which are common to all optical theories, we shall hereafter make use of the electromagnetic theory of light and shall begin by deducing a consequence characteristic of that theory. We shall, namely, calculate the magnitude of the mechanical force, which is exerted by a light or heat ray passing through a vacuum on striking a reflecting ([sect:10.] Sec. 10) surface assumed to be at rest.
For this purpose we begin by stating Maxwell's general equations for an electromagnetic process in a vacuum. Let the vector denote the electric field-strength (intensity of the electric field) in electric units and the vector the magnetic field-strength in magnetic units. Then the equations are, in the abbreviated notation of the vector calculus, $\dot{\mathsf{E}} &= c \curl\mathsf{H} & \dot{\mathsf{H}} &= -c \curl\mathsf{E}\ \Div \mathsf{E} &= 0 & \Div \mathsf{H} &= 0.
\Label[eqn]{(52)}\tag*{\upshape (52)}$ Should the reader be unfamiliar with the symbols of this notation, he may readily deduce their meaning by working backward from the subsequent equations [eqn:(53)] (53).