Page:The principle of relativity (1920).djvu/58

 This equation expresses the law of observation in its most general form. If Φ = π/2, the equation takes the simple form

cos Φ´ = - v/c. 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 A´^2 = A^2 (1 - (v/c) cos Φ)^2/(1 - v^2/c^2). If Φ = 0, this reduces to the simple form A´^2 = A^2 (1 - v/c)/(1 + v/c). 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 A^2/8π is equal to the energy of light per unit volume, we have to regard A/8π as the energy of light in