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
(120)
and
(121)
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
,
(122)
and
,
(123)
§ 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