Page:Zur Theorie der Strahlung in bewegten Körpern.djvu/5

 We denote the aberration angle (the angle between the absolute and relative beam direction) by $$\alpha$$; it is

Finally, it is given by differentiation of equation (6):

an equation that povides the relation of the opening angle of a light pencil in terms of absolute and relative beam path.

If the mirror is moving with arbitrary velocity in the direction of its normal (or also perpendicular to it), then the law of reflection for the relative beam direction is strictly satisfied.

Under "relative" we will always understand "relative to a system moving moving with velocity $$\mathfrak{w}$$. When we are dealing with different velocities of the reference system, we will say in short: relative "with respect to $$\mathfrak{w}$$".

§ 2.

As the positive sense of the normal, or simply as the normal of the surface element of a material body, we want – once and for all – define the direction, which is directed from the body to the outside (into the surrounding aether). By that, also the positive sense of the normal at a radiating or reflecting surface element is given. Now, if the surface element is moving in the sense of the positive or negative normal, then in the following we want to speak in short about its positive or negative motion.