Sentence:  ... though "Gladnesse", and "Griefe", be opposites in "Nature"; yet they are such "Neighbours" and <WSD>"Confiners</WSD>" in "Arte", that the least touch of a "Pensill", will translate a "Crying", into a "Laughing" Face ... 
Meaning of the word marked with <WSD> tags:  A person who lives on the confines, boundary or edge; a neighbour.