Page:Zur Theorie der Strahlung in bewegten Körpern.djvu/12

 Then it is

When we neglect magnitudes beginning with order $$\beta^{4}$$, it becomes

Thus, $$\epsilon$$ is evidently the density of true radiation. In order to calculate $$\epsilon'$$, i.e., the density of the apparent radiation, we have to know the values of $$p_1$$ and $$p_2$$, with which we want to concern ourselves in the following section.

§ 4.

According to, the radiation pressure upon a moving plane is equal to the radiation incident or emanating in unit time (in our terminology, this is the total relative radiation) divided by the speed of light; namely, this pressure is acting in the direction of the absolute propagation in the sense of the negative normal. Since we understood $$p_1$$ and $$p_2$$ as perpendicular pressure components, it is thus:

If we divide away the same factors at both sides, and if we insert for $$\cos\varphi$$ its value from (6), then under consideration of the corresponding sign:

If we insert this into equations (17), then we obtain