Then it is
(22)
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When we neglect magnitudes beginning with order , it becomes
(23)
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Thus, is evidently the density of true radiation. In order to calculate , i.e., the density of the apparent radiation, we have to know the values of and , with which we want to concern ourselves in the following section.
According to Abraham, 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 and as perpendicular pressure components, it is thus:
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If we divide away the same factors at both sides, and if we insert for its value from (6), then under consideration of the corresponding sign:
(24a)
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(24b)
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If we insert this into equations (17), then we obtain
(25a)
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(26a)
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