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This equation expresses the law of observation in its most general form. If , the equation takes the simple form
.
We have still to investigate the amplitude of the waves, which occur in these equations. If A and A' be the amplitudes in the stationary and the moving systems (either electrical or magnetic), we have
.
If , this reduces to the simple form
.
From these equations, it appears that for an observer, which moves with the velocity c towards the source of light, the source should appear infinitely intense.
§ 8. Transformation of the Energy of the Rays of Light. Theory of the Radiation-pressure on a perfect mirror.
Since is equal to the energy of light per unit volume, we have to regard as the energy of light in