In facsimile, a frequency modulationsystem where one frequency represents picture black and another frequency represents picture white[?]. Frequencies between these two limits may represent shades of gray
An intentional frequency change used for modulation purposes
... which deals with his life up to the time of his settlement at Königsberg.
See Quabicker, Karl Rosenkranz (1899), and J Hutchison Stirling, The Secret of Hegel, part ...