The periodogram and its optical analogy

Abstract

I have recently applied the periodogram method to the investigation of several fluctuating quantities, and the experience thus gained has led me to modify slightly the original definition. Having always laid stress on the fact that the periodogram supplies by calculation the transformation which the spectroscope instrumentally impresses on a luminous disturbance, I may now enter a little more closely into this optical analogy, and thus lead up to what I hope will be the final definition. Consider a parallel beam of light falling on a grating, the reflected light being collected at the focus of an observing telescope in the usual way. For simplicity of calculation I assume that the grating considered is of a particular type, which, in a former paper, I have called a simple grating. Such a grating only gives two spectra of the first order.