Without saying as much, Einstein treated these hypothetical plane-wave pulses as if they were monochromatic. He went on to formulate expressions for their energy densities as measured by a uniformly moving observer in terms of the energy density determined by a comoving observer. Expressions for the energy of the two wave trains followed. Like the unspecified body in the related September 1905 derivation, this “plate” could only radiate waves that approximate plane waves, because plane waves are a mathematical idealization. Nor could real pulses be monochromatic regardless of their finite length. Einstein ignored these limitations on the analysis entirely.