The Etymology of Spectre
The 'spectre' is, etymologically, something looked at — from Latin 'specere' (to look), via 'spectrum' (an appearance). Marx's famous opening to the Communist Manifesto ('A spectre is haunting Europe') used the French form. The PIE root *speḱ- (to observe) is among the most productive in English, generating 'spectacle,' 'spectator,' 'species' (the look or appearance of something), 'special,' 'specimen,' 'suspect,' 'inspect,' 'respect,' 'aspect,' 'prospect,' 'perspective,' and 'spy.' Newton chose 'spectrum' in 1671 for the band of colours produced by a prism — literally, what appears when you look at divided light. The British spelling 'spectre' follows the French; American English uses 'specter.'