E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Spectrum, Spectra, Spectre (Latin, specto, to behold).
In optics a spectrum is the image of a sunbeam beheld on a screen, after refraction by one or more prisms. Spectra are the images of objects left on the eye after the objects themselves are removed from sight. A spectre is the apparition of a person no longer living or not bodily present.