Sentence:  ... were certainly the worst and dullest company into which an audience was ever introduced; and (which was a secret known to few) were actually intended so to be, in order to contrast the comic part of the entertainment, and to display the tricks of <WSD>harlequin</WSD> to the better advantage. 
Meaning of the word marked with <WSD> tags:  A pantomime fool, typically dressed in checkered colorful clothes.