Page:Elektrische und Optische Erscheinungen (Lorentz) 133.jpg

 the first medium, and second, we replace t by -t. Then we find, that in 1 the light emerges

when in the paths 2 and 4 the incident motions

and

exist.

However, since the light vector, which is generated by the motion (120) in the first path, has the value

and also the light vector emerging form (121), is to be replaced by

then it is given

From that if follows

and

§ 100. The following remark leads to a simple relation. If we start by a condition, at which the incident light follows path 1, and if we take the mirror image with respect to the second plane of symmetry (§ 96), then we arrive at a condition, at which the light is incident in 2. Consequently it has to be

and in the same way