If we want to introduce herein the absolute beam direction again, then we use the relation immediately given from Fig. 2
And it becomes:
an expression already given by Abraham[1] as well.
Now, by generalization of the thought that was first spoken out by Larmor[2], the same result can be derived from the standpoint of the elastic theory of light, which I would like to show soon.
We consider a light source, moving under the (absolute) angle against the -axis. It is given by
If this wave falls upon a mirror lying perpendicular to the -axis, then a reflected wave is formed, which is given by
Herein, is the angle of reflection.
At the surface of the mirror it must be . If it is moving with velocity in the direction of the normal, i.e., in the direction of the positive -axis, then it must be