Sentence:  ... the notion of the <WSD>microscopic</WSD> was often used in the foundations of quantum theory right from the start, to indicate the realm in which quantum theory is required in order to explain phenomena when classical theory cannot do so; quantum theory was generally considered in those years to apply only to situations involving atoms and smaller <WSD>“microscopic”</WSD> entities... 
Meaning of the word marked with <WSD> tags:  Very small; minute